"Should I ever tell her how many girls I've been with?"
Well... it does, like you probably understand, depend on how many girls that you acctually have been with. And even if it's 5 or 50 girls you've been with it depends on the girl.
If you tell a girl that you've slept with 50 women and she believes in it she'll take for granted that you're a god in bed. But... she will also feel like she, in the future, will be refered to as the 51st girl taken to bed. In other words; She'll feel like you're using her. And that, you don't want.
If you've been with less then, let's say 2-3 girls, it's probably not a good idea to tell her that, as long as they didn't were girls that you fvcked many times. (like in a LTR.)
Now to your case:
You've dated her for 9 months, so you've probably had a lot of sex, and she'll definitely not believe that you're using her.
Damn, you really dated her for 9 months without having the discussion with her?
How many girls HAVE YOU really been with before? How many girls have you kissed? If you've been dating her for 9 months now, you can probably tell her the truth - Even if it's ZERO or HUNDREDS. She trusts you, loves you, and i don't think she cares.
Would you care if she had kissed ZERO or A HUNDRED guys before she met you? I wouldn't.
Get the idea?
