Simply don't pile them because over 100 sketches/cels, the pressure will mecanicaly start to make paintings stuck to sketches.
Prefer vertical storage, an office desk equiped with cartons or archive boxes.
Personnal storage illustration :
Each sheet or cel covered with a Melinex 75microns transparent cover that is in use by museum restaurators to protect paintings from acid ($5 per cover). For safety reasons I always remove the plastic bags from Mandarake or collectors as their non acidity can't be confirmed.
Expensive arts (over $500) are double protected with a non transparent anti-alkalin paper sheet ($3 per sheet).
I'm not really a fan of portfolio because they are foldable, and when I've to use crowded suburbs train to bring arts from a place to another, bring such box with me avoids from a lot of stress, like walking with a cute micro-safe

Additionnaly write "acid free" on a portfolio does not mean "lignin free", portfolio are made for sketches but normally not for acrylic paintings.
So I prefer use professional restaurator regular material to protect collected artworks.
Compare to an anime cel cost, this kind of long term protection is really not expensive because you've to split its cost on more than 10 years
