It's a tossup.
Many people ask questions that they do not REALLY want to know the answer to. This behavior begins in childhood. I still remember my children asking me things like if the Easter Bunny was real. I always looked at them with a serious expression and asked, "DO YOU REALLY WANT TO KNOW?" If they were really ready for the truth they pushed the issue. However, if they were not, the just changed the subject and it was all good.
I have never lied to my children and I think it is becuase of this that they volunteer all kinds of stuff to me even though THEY KNOW that I won't be thrilled with what I am hearing. Because there is trust.
I asked my last lover how many women he had been with (I am not jealous at all.) and his answer was that he honestly could not BEGIN to count. He didn't know. He is nearly 42 years old, has status, money and is extremely attractive and there was a very large period in his life where he slept with whomever he wanted, whenever he wanted. But I do trust that that was then and this was now... We have not seen each other in a very long time as distance and circumstances have kept us seperate. Will I ask him if he has been with any other women? NO. Why? Because it doesn't matter to me in the grand scheme of things, but I would be lying if I said that it wouldn't hurt me deeply if I KNEW for a fact.
He once said that though he knew he would not be my first, he would very much like to be my last. A romantic notion indeed. However, there is no spoken commitment on either of our parts so "Don't ask, Don't tell" is the best policy here for us.
So just consider the lady in question and what her motivations are in asking. BUT DO NOT LIE. You don't have to give an exact number, but, you shouldn't lie and say that you didn't "live the life" for a while. If she is so insecure as to be incapable of handling the fact that there were women before her then perhaps she is not someone that will be strong enough to handle the difficulties that life will inevitably hand you as A COUPLE.