to cover the various requirements at each school, you should get 3 letters from profs that have taught you: 2 science + 1 non-science
plus a couple letters from your meaningful extracurricular activities (e.g. research, volunteering, etc.). these are usually not specifically required by med schools, but help give a better picture of you.
some schools, like uc davis or tulane, don't want more than 3 letters or something like that. but in general, you probably wouldn't want to send more than 5 letters to a school.
i also recommend asking 1 more prof than you need letters. e.g. i had one science prof flake out on me, so it was a good thing that i requested letters from 3 science profs.