Quality over Quantity
10 pieces is a good start, any more than 25 might make you seem indecisive
Respect your reviewers time as they have to go through hundreds of applications
Some work may not be appropriate(works created by following tutorials)
Favor more recent work
Having old work in your portfolio may make it seem like you don't make art often
Keep in mind your portfolio may be judged on your worst work
If applying to multiple fields, having multiple portfolios with works appropriate to their fields might be good
First impressions matter but don't want your worst work as their last impression
Recommended to have your two best works at the beginning and end
Simple Neutral Background
Don't divert attention from your work
Be consistent with spacing
No watermarks
Signature in your art is acceptable
About Me/Biography
Resume/CV
Contact Information
Put them at the bottom of every page if you have to. People should know how to contact you.
Choose a standard, easy to read font. Readability is more important than looking good or fancy.