This is never a good thing to ask. Nothing positive can come from it because you'll never be satisfied by the answer. Say she's racked up a score of partners, obviously that would bother you. But then if she's only been with a handful, I'm sure you'll at first be okay but then once again, your jealousy issues will arise and you'll want to know more about those few and begin prying for something else. For me these days, if I trust a girl I don't care how many she has been with.