I had quite a few, but they were all pretty much needed. One from a community college instructor (required for some schools if you attended a CC). Two science profs, one non-science prof, and two PIs (again, required at some schools). Several of those were assembled into a packet/committee letter. I entered it in AMCAS as three letters, and I'm guessing most schools viewed it as three letters (unless they read everything in the packet).