I'm also not sure how it is for knitting or crochet, I can do neither, but you need to think about your turn around time to make it when you list a quantity of something. I can feasibly list 18 Medusa rings, not because I actually have them made, but because the rings are already on a sheet saved on my computer and I can just print it out, bake, and roll within half of an hour. So I don't really have storage issues (rings are also small) but I'd think making something knitted would take far longer, which means saying in the listing that there is a two week production time or something.
But yes, I'd stick to smaller numbers, that way you also just aren't obligated to keep that much on hand.