I remember during high school when my friends would ask me the question and I'd emphatically responded, “Ten, bitch!”
That was an okay answer in high school because we were gonna get drunk off six beers and pass out at our friend's house the majority of the time anyway, but nowadays it's typically a woman that's actually interested in the answer who’s asking the question. Answer correctly, she could be even more attracted to you, answer incorrectly or awkwardly and you're going to immediately demonstrate that you're not comfortable with the answer.
I heard some good responses from advice columnists like “I've been with enough girls to know that no good can come out of answering that question,” or answering with a joke like, “I'm too mature to keep track of the actual number,” which are both decent responses, especially if it’s your fuck buddy or some girl you just met. But if I'm hanging out with a girl I actually like I'll use it as an opportunity to show the girls she's dealing with an intellectual mother fucker.
I'll drop some knowledge on the broad and something like, “Personally, I don't think it matters the slightest bit how many women I've been with or how many guys you've been with either. The only thing that matters is that you're honest and upfront with the person you’re involved with and you're not doing shit behind each others back. And I have no idea what's going to happen with us because it's early, but if we’re still hanging out in six months or a year and you really want to know the true answer to the question, ask me again and I'll tell you straight out.”
When a girl hears a response like that she's typically taken back and will either straight up agree with you or tell you that she understands what you're saying even if she might not agree. It’s a win-win situation.
If it’s a casual question give it a casual answer. If the chick is serious, show some sack and drop some knowledge on her.