20 stones exist
2: 1-2 level spell slots
8: 1-4 level spell slots
12: 1-6 level spell slots
16: 1-8 level spell slots
20: 1-10 level spell slots
EX: 13 stones. 1(6th) + 1(5th) + 1(2nd) =13
The limits
Each level will acquire more levels to recover than before with certain number of stoned based on the lowest level you acquire the spell slot