awww.... cmon now! This is very highly individualized. First of all, studies are not reliable since people don't disclose information so the studies are relatively inconclusive. To make matters worse, the younger population probably has less actual intercourse but more sex where penile vaginal penetration doesn't take place (oral, masturbation, etc.). They avoid intercourse but they still have sex. Although they might not believe they are having sex. There's even a cosmetic process of revirginization. It's an interesting question but I think not only will you find a very large regional difference with much higher rates in urban areas vs. rural but even those numbers could be misleading due to what many today consider "actual sex".
Even if a girl has had many partners, that doesn't mean that today she is a slut. And a girl who never had sex until she was 25 may be painting the town. Guys are much the same.
The one thing I DO see a lot today with some of my younger girls is that young women today have difficulty staying OUT of relationships. This may be due to peer pressure where her girlfriends tell her "You got to get back out there girl!" or it may be that they have a fear of being alone, societal pressure, etc. etc. Men generally don't have this problem to such a degree. A guy is more likely to find his way out of a bad relationship and just be single for a time while a girl will simply float from guy to guy. This is a huge point of frustration for me. Especially when you have young girls who really need to find the things in life they enjoy and being in relationships that require so much maintenance, they never seem to find themselves. So being lost, the moment they become single, panic sets in and they bolt right back into the arms of the next guy. I think this is probably more dangerous than the actual number of sex partners.
Ask yourself if the girl has hobbies. Is she dedicated to anything? Is she a member of any groups (social, athletic, etc.) If you get a canned answer like "Well, I like long walks on the beach!" than I'd say be cautious. If she says she’s a member of the Young Professionals Organization in her area or plays volleyball, or any of the MANY things available to people, then perhaps you've made a discovery. Now before you flame the hell out of me, those were just examples. A girl can like long walks and at the same time... forget it. You know what I mean. Of course all this is moot if you discover she has a penis...
Anyway, there is a lot more to it than meets the eye. I suppose this is my point ultimately.
The last thing I'd like to mention is that the amount of STD's has reached epidemic proportions. I've read reports that in some areas as many as half of women are carriers of papilloma virus (HPV) which men can also be carriers and not have symptoms. In women this can lead to more severe health consequences. So I'm just saying be very careful out there boys and girls. You may find your way out of the relationship only to find that you've been given a gift that keeps on giving.