Well fear no longer friends! I think we can tackle this one.
First of all, I believe that you've gotta know what kind of girl you're dealing with so that you can try and assess why she's asking this nonsense. If you can understand why she is asking this, then you'll know how to tackle the question when it comes your way. The way I see it is this question is a shit-test of sorts, designed to give the girl a good read on what your personality is like and what your intentions are for her. This could be good news for you, cause it means that she's likely sizing you up as potential boyfriend material. On the other hand, if you're aiming for girls to categorize you as a seducer, this is a clue that you're not quite there yet. Afterall, do you think a girl would ask Tommy Lee that question? No, no they wouldn't...
So aside from all that, let's say you've gotten the question, now what are you supposed to say?
If your skill level is something like beginner to intermediate i'd recommend the following:
Your first option would be telling her something like "Oh, um... I don't know, like a couple hundred maybe? Three hundred? Not really sure." The idea here is to say something believable enough that she's gotta think about it, but absurd enough that she knows it's not true for you. The combination will kind of tease her for asking the question at all.
After that the girl's probably going to ask the question again somehow, so i'd move on to a "you want me" frame. It'd go something like "Look at you, you're like soooo into me right now. You wanna know like every little detail about my life. That's cute. Well be patient little girl, soon enough you'll be able to drag all of this stuff out of me, I promise, but we're not quite there yet."
After this she'll likely subside for a bit, a few days if you're lucky. At that point I would put it off again in the same manner until later when you feel comfortable enough with the girl to actually tell her if you choose to do so. The timing for this is loooong after you've slept with her i'd say.
If you're skill level is advanced, i'd recommend something a bit harder like this:
"Are you serious? What are we in like high school again?"
Then if she presses:
"To be honest, I really have no clue. I quit counting a long time ago."
With the beginner/intermediate option you'll buy yourself some time to continue to game the girl until you really feel more connected with her and comfortable enough to be honest about what your past has been, and that's if she even presses the issue. With the advanced option, you're really shooting for a seducer vibe and setting up and scumbaggish/player frame. You don't care what she thinks about you or the sex you've had. This means that by continuing to talk with you, the girl implicitly has decided that it's not a problem for her that you have had sex with (and might have sex in the future with) a lot of girls.
The reason I list both of these options rather than just the advanced option is that if you just say the advanced option and you're really congruent with being that guy yet, the girl's going to know it and it'll come off like you're an arrogant guy who thinks he's cooler than he really is. So really use what option is best for you, and if you're moving towards the seducer frame then start to work this harder stuff into your game when the time's right. Either way, knowing this information will get you past this bullshit question in a way that maintains or increases the girl's attraction to you and lets her know that you're not about to be phased by her little ploys to rattle you up.
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