A few questions about the cel and where it lives, and you get a storage method with its trade-offs stated. Based on the Animation Cel Preservation Handbook.
“Only 15–20 cels” is not a safety limit. What matters is pressure at the contact patch, and paint is not flat — contact happens on raised ridges spanning a few square centimetres, so a small mass concentrates into a surprisingly high pressure.
An 11×14″ page in a polypropylene sleeve — cel, sleeve and black paper —
weighs about 84.5 g. Half the weight of the binder shell (0.284 kg) is taken as
resting on the stack, so the mass above position n is
0.284 kg + 0.0845 kg × (n − 1).
Pressure is that weight over the contact area: P = F / A.
Contact patches on painted cels typically run 2–10 cm². The 5 cm² default reproduces the handbook's published pressure column; drop it toward 2 cm² for heavy raised paint or thick ink lines, since less contact area means more pressure, not less.
Vertical storage takes this to zero — gravity acts along the cel's edge instead of into the paint. This calculator describes horizontal stacks only.
Search a term from a product listing, or browse a category. “Acid-free” describes a material's pH the day it left the factory, not a permanent property — a porous acid-free board next to a cel absorbs off-gassed acid until it becomes a source itself.
Nothing matches that term. Try the material name from the packaging — manufacturers usually state the polymer or board type somewhere.