My roommate had absolutely none when he applied and got into a number of programs. I had a pretty good amount, but that's because I decided upon MSTP fairly late. In any case, it's always a safe bet to have some, but you shouldn't worry about having alot. Also, not having any is not going to exclude you, and may not even hurt you.
Someone may point out a program where the MD admissions has a significant say or must admit you first. I've seen first hand where the MD committees say "It's okay they don't have clinical experience, they have alot of research and they're going for MD/PhD. No problem." Of course this won't work for ALL schools, but I think this general advice holds.