I am a bit lost with your post.
With all these doubts and with all the info you have about her why are you choosing to stick around?
It's only been 7 months.......why not get out NOW?
It sounds like you want to prove something or win an argument, and that you think that if you do, somehow things will change. You can't argue a flirty person out of being flirty. You were attracted to her for her beauty and flirtatious nature yourself, and now she has to stop because you realize she has sometimes taken it to its logical conclusion? It doesn't sound like you think she is cheating on you, it just sounds like you're bending yourself into a pretzel wondering who she might have slept with before you came along. That is a waste of your time and emotional energy, unless you are a lover of high drama.
Given that it all bothers you so much, why not walk away and be glad, because you dislike having a girlfriend who attracts so much attention? Oh, yeah, you don't want to, because she is pretty and sexy. Sorry, you can't have it both ways. Find a quieter girl to fall in love with who does not have such a history.
Incidentally, for what it is worth, you seem to think you will win an important moral victory if you get her to admit the number is bigger than 8. But, this thing you think so important, really isn't that important to everyone. Maybe your girlfriend got bored with the question long ago (or tired of being judged over it) and just settled on saying 8. Regarding you feeling threatened by the possible number (or more pertinent, thinking it means something about how she might behave in the future), I think you really need out of the relationship because it matters to you that much.
In the meantime, be ultra careful not to have unprotected sex. The last thing it sounds like you need from this girl is a baby.
She is trouble. So leave her and be done with it. However, you are not going to leave her, So deal with it. Just have fun with her and don't take the relationship too serious. She will break your heart at the end if you are not careful.
I think when you describe her as lazy, needing men to support her and her child, going from relationship to relationship to do that and lying . . . not really sure what is worth sticking around for. She must be really hot. LOL
Move on and save yourself the heartache. good luck