A fundamental rule in the Pokémon TCG is that you can only play one Supporter and one Energy per turn. Playing too few Supporters or Energy in your deck causes you to miss out on those precious once-per turn opportunities, while playing too many leaves you flooded with Supporters and Energy that rot in your hand (when they could have been other playable Trainers or Pokémon instead). What is the sweet spot for Supporter/Energy count?
There are Supporters that draw/dig/search in order to find you the cards you need, and then there are Supporters that aren't any of those, and are more like "effects that would be too powerful if printed as an Item" (e.g. Boss's Orders, Professor Turo's Scenario, Briar). For the rest of this section, whenever I mention "Supporter", I'm referring to draw/dig/search Supporters unless stated otherwise.
I count Ciphermaniac's Codebreaking as a Supporter, even though it isn't plus in card advantage. I also count Pokégear 3.0 and Secret Box as Supporters, even though they are technically Items, and Pokégear 3.0 may whiff.
The typical number of Supporters is 6-12, and most decks play 2 Boss's Orders on top of that. The lower your draw/dig/search Supporter count, the more non-draw/dig/search Supporters you can play. High Supporter count decks tend to prefer going second, so they can play those Supporters on their first turn.
Basic/Stage 1 decks usually play fewer Supporters, but they will have plenty of supplementary draw/dig/search on Abilities, like Mew ex, Squawkabilly ex, or the Noctowl engine. Stage 2 decks usually play more Supporters — Arven is a common one, as it can set up two Stage 1s on the Bench in 1 turn with Buddy-Buddy Poffin + Technical Machine: Evolution.
Noctowl decks can get away with low Supporter counts, as Noctowl can search for Supporters when they're needed.
Tera Box and Flareon ex run 5-6 Supporters with Mew ex and Noctowl.
Raging Bolt ex runs 6-7 Supporters with Teal Mask Ogerpon ex and Noctowl, optionally Squawkabilly ex or Mew ex.
Gholdengo ex runs 6-8 Supporters with Genesect ex, which is almost like having a free Jacq every turn. Gholdengo ex itself draws cards.
These decks run Carmine, Professor's Research, and Squawkabilly ex to draw through their decks quickly.
Ceruledge ex runs 7 Supporters with Squawkabilly ex and Revavroom.
Miraidon ex runs 7-8 Supporters with Squawkabilly ex and Mew ex. Miraidon ex itself searches for Pokémon.
Armarouge Box runs 8-10 Supporters with Squawkabilly ex and Mew ex.
These decks have good draw/search engines.
Charizard ex runs 8 Supporters with Pidgeot ex.
Conkeldurr runs 8 Supporters with Dudunsparce and Enriching Energy, sometimes also Pidgeot ex.
N's Zoroark ex runs 8-9 Supporters. N's Zoroark ex itself draws cards.
Dragapult ex runs 10 Supporters. Drakloak digs for cards.
Gardevoir ex runs 11 Supporters with Mew ex. It does not play Boss's Orders, as Scream Tail can snipe the Bench.
Marnie's Grimmsnarl ex runs 12 Supporters and usually no Pokémon with draw/search Abilities.
Ethan's Typhlosion runs 12 Supporters with Pidgeot ex, Drakloak, or neither. Ethan's Quilava can search for Ethan's Adventure.
These decks usually play 4 copies of each of the above cards, and have zero or limited draw Abilities. If you find yourself stuck with only Arven for the turn, you can search for Pokégear 3.0 to dig for a better Supporter next turn.
Joltik Box runs 11-12 Supporters with Miraidon ex and Mew ex.
Crustle runs 18 Supporters (including 1 Lana's Aid in that count) and no Pokémon with draw/search Abilities.
Decks have varying Energy requirements, and hence also varying Energy counts. The typical Energy count is 5-14, but note that this number may be deceiving, as decks often have ways of recovering Energy from the discard pile. Super Rod is the most common way of inflating the Energy count, effectively adding as much as +3 Energy while only taking up 1 deck slot.
Some considerations when deciding on Energy:
If you play Munkidori, you'll need Darkness Energy (or Luminous/Prism Energy) for its Ability.
If you splash a color (e.g. aforementioned Munkidori in a non-Darkness deck, or Teras with 3-color attacks like Eeveelutions or Pikachu ex), you'll want at least 2 Energy of the splashed color. Even if you only ever need 1 Energy of the splashed color for the duration of an entire game, it may end up Prized, so play 2. Sparkling Crystal can count as the 2nd Energy of multiple splashed colors, compressing what would take up several deck slots into one.
A method that I use for determining preliminary Energy counts is to imagine a game where your opponent lets you set up, and does nothing to waste your Energy (like Bossing in something and making you pay for its Retreat Cost, or using Crushing Hammer or Handheld Fan) beyond KOing your attackers. How much Energy do you need in that idealized game? This includes Energy for setup attacks like TMs or Joltik's, for Munkidori, and of course for your attackers, which, if they are all 2-Prizers, you'll need to provide for three of them. Once you've calculated that number, add 3 to it OR a Super Rod to your deck, and that should be a good first guess.
Conkeldurr runs 4 Energy. Conkeldurr itself doesn't need Energy to attack (it relies on Brute Bonnet Poisoning it to attack for free), so it can afford to spend Energy attachments on Energy cards with when-played effects: Jet Energy to switch, or the ACE SPEC Enriching Energy to draw (which can be reused after Dudunsparce shuffles it back into the deck).
Ethan's Typhlosion runs 5 Energy, provided it doesn't have secondary attackers. Builds with Charizard ex or Dragapult ex will run 7-8.
Charizard ex runs 6-7 Energy (5-6 Fire, 1 Mist, +1 Darkness/Luminous if using Munkidori). It plays 2 Super Rod, as it might need to recover the thin 2-1-1 Dusknoir line in addition to Energy.
N's Zoroark ex runs 6-7 Energy, +1 Fighting if they are on Cornerstone Mask Ogerpon ex, or +1-2 Reversal if they are on N's Darmanitan. The number of copies of N's PP Up is also up (pun intended) for debate, with builds ranging from 0 to 3 copies, and those playing more PP Ups being more likely to run only 6 Darkness Energy.
Dragapult ex runs 7 Energy, one of which is Neo Upper, so the amount of Energy provided would be 8. Two attacking Dragapult ex's and a final Bloodmoon Ursaluna ex require a total of 5 Energy, so you have a decent amount of wiggle room when it comes to budgeting your Energy. A retreat to Budew early-game or a Luminous Energy for Munkidori might tie up more Energy. The deck usually doesn't play Super Rod, but it does run a fairly high count of Night Stretcher (3-4).
Marnie's Grimmsnarl ex runs 8 Energy. It's very similar to Charizard ex, since it accelerates Energy from the deck and its attack costs 2 Energy. The main difference is that Marnie's Grimmsnarl ex also relies on Munkidori supporting from the Bench, so its upfront Energy requirements are higher — a board of one fully-powered Marnie's Grimmsnarl ex and two Munkidoris requires 4 Energy, compared to 2 for a single Charizard ex. Thus, it runs 8 Darkness Energy + 1 Super Rod, versus Charizard ex's 5 Fire Energy + 2 Super Rod.
Many decks with 3-Energy attackers play Crispin: Tera Box, Flareon ex, Joltik Box, and Crustle all do. Furthermore, Sparkling Crystal could be considered an honorary Energy: reducing attack costs by 1 is functionally identical to a rainbow Energy, but even better because 1) it uses a Pokémon Tool attachment (which is unlimited) instead of an Energy attachment (once per turn), and 2) it lets you get away with shuffling/discarding only 2 Energy with Wellspring Mask Ogerpon ex's or Pikachu ex's attacks, meaning you only need to get 2 (not 3) Energy onto them to use their attacks again (which is easily accomplished with Crispin).
Tera Box runs 9 Energy + Sparkling Crystal. It has a heavy Grass count for Teal Mask Ogerpon ex, then the remaining types of Energy are 1- or 2-ofs. Psychic Energy is a 2-of in case Lillie's Clefairy ex needs to KO Dragons or do a Terapagos ex impression; it's not a Tera, so it can't make use of Sparkling Crystal.
Flareon ex runs 9 Energy + Sparkling Crystal. Its color requirements are very demanding, as both Flareon ex and Sylveon ex have 3-color attacks, and Leafeon ex uses a splash color. There are 2 Basic Energy of each of the 4 main colors: Fire for Flareon's attacks, Water and Lightning which Flareon's and Sylveon's attack costs share, and Psychic for Sylveon and Lillie's Clefairy ex. That leaves 1 Grass Energy for Leafeon.
Armarouge Box runs 11 Energy (7 Fire, 3 Prism, 1 Legacy). It is a toolbox deck, so it has some attackers that require rainbow Energy. Fortunately, their attacks are not color-intensive (other than Roaring Moon ex's or Munkidori ex's), so 4 rainbow Energy is enough to power the same attacker thrice (using Night Stretcher to recover it if it gets KOed) with 1 to spare. If you're out of rainbow Energy, you could always fall back on your big damage Fire/Colorless attackers like Reshiram ex, Gouging Fire ex, Terapagos ex, or even Mew ex.
Decks in this category have ways to recover Energy from the discard pile to power up repeated attacks: Gardevoir ex's Ability, Superior Energy Retrieval and Levincia in Gholdengo ex, Professor Sada's Vitality and Energy Retrieval in Raging Bolt ex.
Gardevoir ex runs 10-11 Energy (7-8 Psychic, 3 Darkness, +1 Darkness if they are on the full 4 Munkidori and TM: Devolution). Generally you'll need 6+ Energy to start one-shotting things; 0-2 on Munkidori, then the remainder on Scream Tail/Drifloon. Attaching just 2 Energy on Scream Tail/Drifloon is not enough, as you also need damage counters on them to pump their attacks, which you get by using Gardevoir ex's Ability.
Gholdengo ex runs 11 Energy: 3 Metal to pay for attack costs, 2 Lightning for Levincia, and 1 of each of the remaining 6 types as Energy Search Pro targets. While Make it Rain nominally only costs 1 Energy, you need to discard Energy from your hand to pump it; 7 Energy in hand would hit Stage 2-one-shotting numbers.
Raging Bolt ex runs 11-12 Energy. Its color requirements are fairly demanding: Teal Mask Ogerpon ex's Ability needs Grass Energy, and Raging Bolt [ex] requires Lightning and Fighting for its attacks. The amount of Energy that Raging Bolt ex needs can be high too, 4-5 Energy on board to KO opposing ex attackers. A board of 2 Raging Bolts ready to attack requires 2 each of Lightning and Fighting Energy, so playing a 3rd copy provides some safety in case one Energy of either color is Prized.
Technically, Ceruledge ex falls in this category, but its damage scaling is really bad. For comparison, each Energy is worth 40 damage with Scream Tail, 50 with Gholdengo ex, 60 with Drifloon, 70 with Raging Bolt ex, but only 20 with Ceruledge ex. For this reason, it needs a lot more than 10-12 Energy.
These decks play Crispin. Going second is preferred with these decks, as your Supporters (Crispin, Arven for Earthern Vessel, Pokégear 3.0 for either) represent more outs to hitting the T1 Energy drop.
Joltik Box runs 13-14 Energy. It needs to hit its T1 Energy drop for Joltik's Jolting Charge. It splashes colors (two, in fact): 2 Metal Energy for Pikachu ex, and 1-2 Psychic Energy for Lillie's Clefairy ex. Only 1 Psychic Energy is OK, as you can keep Lillie's Clefairy ex Benched and attack for Weakness with Mew ex (whose attack costs 3 Colorless) instead.
Crustle runs 14 Energy. It needs to hit its T1 Energy drop for Dwebble's Ascension or Technical Machine: Turbo Energize. The deck runs 3 types of Energy: Grass for Crustle, Darkness for Munkidori, and Fighting for Cornerstone Mask Ogerpon ex. Fighting is the least important color since there's only one Ogerpon, but they still play 2 Fighting Energy, not 1 — even if you don't need Ogerpon, you can grab Grass + Fighting off Crispin to charge Crustle, and leave Darkness for future Munkidoris.
The high Energy count in these decks means you are likely to draw into hands with multiple Energy, but they invariably use Squawkabilly ex, Professor's Research, Carmine, and Ultra Ball to discard them for more useful cards. They also have cards that benefit from having Energy in the discard pile: Magneton/Eelektrik in Miraidon ex, Ceruledge ex itself in its own deck.
Miraidon ex runs 15 Energy. It needs a high count to improve Electric Generator's consistency.
Ceruledge ex runs 18-19 Energy. Like Gholdengo ex, its attack only costs 1 Energy, but requires you to jump through hoops to boost its damage output, the "hoops" in this case being getting Energy into the discard pile. Brilliant Blender is the best at doing this, milling up to 5 Energy at once.