There isn't a set number. I would say between 15 and 30, but it depends on the type of presets in the pack, and the amount of time it takes to make the pack.
I don't like packs with a whole assload of presets in them where one preset is just a slight edit of another. make a whole assload of presets but only release about 15 of the best. This way you can see what your strong points are and work on other points. Also if you're making a lot of presets you're getting better naturally and this will show in your work.
Uhm.. yeah... uhm... yes yes... I don't think 30 is too much, but when you go over 50 it's an assload...
Anyway, how about posting presets in the AVS PRESETS forum? 😉