I had a total of 7 that I sent to all the schools I applied.
My supervisor sits on the admissions board for UF college of Med. She thinks the letter is more important than interview, volunteering, and all that crap. A letter is the only way for the admissions board to learn about you as a person.
with that said...
Admissions can EASILY tell if your recommender has a letter saved in a format and just inserts your name into it. They scan through and look for personal info and antidotes that tell about you. So if you are confident you will get a strong letter send in as many as you want. But if your letter is a stretch, you didn't really know them, or something on those lines it really wont help your application that much. My sup says she throws letters that are formatted out of consideration, just some food for thought.