I could easily do with about 5 bottles, as long as I have one for each major type of occasion. I think it's only normal to get sick of your favorites over time. It's also good to have a non-perfumey pallet cleanser, like Angeliques sous la pluie.
Of course, the more the merrier, to a certain extent. I think more than 15 is a little overboard in terms of practicality. Under 15, you can rotate quite a bit and still get to know and appreciate your fragrances. Not to say I wouldn't love a collection of 100+. I'm truly envious of some of the wardrobes on here.