Every school is different, but usually it's at least two from science professors. I sent in two from science professors and one from my PI (from the lab where I did research). Oh, and one from a dentist, but I don't think this is required by every school...I can imagine it helps though.
Generally, at least 3 from your professors and one from a dental professional. Although at loma linda i had to send in two extra letters in addition to those 4.
I had one from the health advisory committee at my school and one from another science professor and that did the job for me at all the school i applied to.