Sorry to burst your bubble, but no, there is no magic number :( As others have said, some shops with very few items can be doing great and others like mine have over 100 (around 150) and it doesn't seem to make enough difference for it to be called a 'magic number'. However, I do believe that for some things it does help to have a larger selection of items. There are factors to consider, what's the item's main purpose, aesthetic or functional? If it is mainly for decorative purposes, a larger selection is very important as tastes differ significantly. If it was mainly functional (with a craft or art involved, of course) maybe just a few listings would be enough. I'm thinking also of In-N-Out who is successful with a small menu...
Anyway, I don't like 'talking' too much, and I'm starting to, but I hope these ideas help...