Glamour plates are functionally the same thing as gear sets, a text file with a list of gear items and their dyes. Since there's no materia, they actually have less data to store. The gear set limit was drastically increased to 100 in Endwalker, so server storage clearly isn't the problem.

I'm no developer, but I'm pretty sure the reason we still only have 20 plates is simply the UI - because they designed it with rows of numbers at the top rather than a future-proof list which could be expanded like the gear set menu. The UI guys made an oversight when designing the menu, and I guess they just don't feel like redesigning it.

That appears to be the reason at least, CBU3 is free to correct me.