First, all med schools prefer to have a recommendation from the pre-med committee at your college. This committee rec is a composite of individual letters from your professors and also the thoughts of the committee member who interviews you before you apply. I'm sure the number of individual recs that the committee needs to write a letter varies from school to school; at my college, we needed 2 science and 2 non-science recs, along with any from extracurricular sources. If your school does not have a pre-med committee, you'll have your professors send the recs directly to the med school. Again, it will depend on the med school, but I think to be absolutely sure, you should count on 3 science recs and 2 non-science, though most schools probably don't require that many. Of course, that doesn't include recs from non-academic sources (research, community service, etc).