I am not sure numbers matter as much as the situation behind the numbers. A girl who has done things like wave to a random guy in a car while driving down the road, and end up having sex with him without ever knowing his name is likely to do impulsive junk like that again. A girl that cheated multiple times on a previous boyfriend is probably more likely to do that with you.
Due to my current situation I get to be around ALOT of pregnant girls including seeing their vaginas in all sorts of situations. Although it is not 100% true, I think more often than not the better looking the girl the more likely she is to put out. The more she puts out the more loose and stretched out she is. I would not have believed this before, thinking that a vagina is stretchy and will resume its previous shape - although having a baby loosens it up a bit - but now I have to say its not true - if the girl is getting alot of action, even if she has not had a baby yet, she will stretch way out. You can often park a semi truck in the 19 year old girls with very extensive sexual histories.
Almost invariably, if the girl is a real looker her medical history will show 5 or 6 TAB's (therapuetic abortions) plus many instances of chlamydia, syphillus etc. I don't know if it is because more guys hit on the better looking girls or if the better looking girls spend alot of effort looking good so that guys will hit on them..... but I think the best odds of getting laid (for the young guys) is with the good looking girls. WIth young girls - hit on the lookers as she is most likely the easiest - although loosest snatch.
Past can matter - if they have impulse sex in the past, or have a tendency towards self destructive behavior they will probably do it in the future too. And if they have been very very sexually active they can be as loose as a woman who has had 4 or 5 kids.
I met one girl not too long ago - GREAT looking. Told me how when she was 18-19 she and her roomate use to sneak into bars and have contests on who could get laid. I told her that was the stupidest thing I hade ever heard - it was like (to borrow from Chris Rock) like playing basketball with a retard and then bragging about how you won. I find it so funny that such a beautiful girl would need to prove to her ego that she is attractive by going out of her way to get some drunk guy in a bar to take her home to have sex. A girl like that will always be easy prey - luckily for drunk ugly guys everywhere. But if you got into a relationship with her and were not constantly feeding her ego, she might sleep with any guy who massaged her ego a little bit just to make herself feel attractive.