I would think it would have to be taken in context both of age and how the hookups happened. I'm not sure exactly when I'd be bothered, but it's not a pure number. If I'm 25 and dating a 23yo who had 10-15 partners in college, that's different than if I'm 35 and dating a woman who has had 10-15 partners via serious and semi-serious relationships over the years. Even then, in the former case, it's going to be less about being "judgy" and more about wondering whether the girl is actually in a place where she's going to want to settle down. I think I'd also be more concerned about recent promiscuity than past stuff....and by promiscuity I mean a significant number of hookups outside of semi-serious relationships.
But it really can't be useful as a pure number. A number of 6 might be pretty high for a college underclassman....but it's going to be pretty low for a woman in her early 30s.