She's sensed that you're trying to hard to impress her with the "I'm equally as sexually adequate as you. You ain't nothin special" attitude. Why even bring it up? Just have fun with her. If she asked me a question like that. I'd brush it off and reply with some shitty jokey line.
Her: Exactly how many girls have you slept with.
Me: My fair share.
Her: Well you're either a casanova or you're exaggerating.
Me: Casanova eh? hmmm I definitely look like David Tennant so yeah Casanova I'd say
(David Tennant played Casanova in a tv series. It was awesome)
Don't act like you're not reacting to her. STOP REACTING TO HER. Just have fun with it. Who gives a shit if she fucks off if she's "more experienced"