I agree that I don't think it is an actual number but fills in a category. If you attend a lot a dinner parties or functions, maybe you want a few more clutches.This varies from person to person. For me, I think it's 10 BUT I have every category covered. If someone had 10 Seedy's, I don't think they would have enough.
I think every woman needs:
An everyday fall/winter bag
An everyday spring/summer bag
An evening bag
A crossbody bag
A beater bag
A travel bag (can be the same or different as beater bag)
For me personally, I want to be able to have something from each category AND be able to use all my bags on a regular basis.