it's in the past, but i also think it depends on your age.. if you are a younger woman (between 18 and 24) and you've had that many different partners, are you really ready to slow down and be monogamous? that would be my (as a guy) number one concern.. i don't think that it would necessarily be a deal breaker, but it would be in the back of my mind.
definitely don't tell him a lower number and then later in life tell him the real number, because that would create a BIG trust issue and then it maybe would be a deal breaker.. either tell him, "does it really matter, i'm ONLY with you now, and that's all that's important," or tell him the truth.
and if it is a deal breaker, then obviously he wasn't right for you.