First check to see if your undergraduate institution has a pre-health committee. Med schools will want a committee letter if your undergrad offers that, or else you'd have to have a good reason why you don't have a committee letter.
My school's committee asked for 5 LORs and they claimed that those covered all the bases for any school. It was at least 3 science LORs, one of which could be replaced with a research LOR, and the remaining 2 should be "character letters" (aka from ECs). But to be sure, do what Cinclus said and check the specifics for the schools you're applying to.