Does anyone remember the old Mary Tyler Moore show when Mary had some sort of discussion with her boss, Lou, about "the number?" If I'm remembering right - according to Lou "the number" should be "3." More than that and you were a bad girl!
Personally, I think it's not so much the exact number that matters; it's more the context of the former relationships. Although I agree with KStiletto and TheWriteAlex that super-high numbers don't say anything good about a person.
But by a certain age, I'd also be concerned if the number were "zero." How do you get to be age 30, say, and never been in love enough to have had an intimate relationship? It's possible, of course, but I'd like to know why. If it's due to sincerely-held moral/ethical reasons, fine. If it's because the person is scared of sex and intimacy, that might be reason for concern.