For some people, it's an easy discussion because there is no issues with things like retroactive jealousy or feeling like their partner is...ahem... "used", for others it's felt to be a required topic for whatever reason and then sometimes they allow it to eat away at their relationship - be it retroactive jealousy or the aforementioned "used", or simply to see if the other person aligns with their outlook on it all - which can also cause the relationship to fall apart due to differing morality, values, what have you.

If you genuinely feel that the past is the past and have no desire to allow it to dictate your future, then I would suggest not talking about it. To what end would it serve in this case? You already know she has slept with more people than you have.

I would just tell her that you would not be comfortable with the topic and thar the past is the past and you aren't going to judge her for it.