1. Look at school limits. Some don't want more than 3 or 4, and are specific about the sources of those LORs
2. Do those letters give additional perspective on you? I've seen a good breakdown as 2 science faculty, 1 non-science faculty, 1 research, 1 clinical.
It depends. Some schools take a max of 3, some take a max of 10. You should apply LORs as necessary. I generally gave the maximum that a school would accept.
Make sure not to check only on hard limits, but also on recommendations from medical schools. Some say something like: "We'll take up to 5, but we do not recommend that you send more than 3 unless you feel that any additional letters can provide something meaningful that the others cannot." In that case, you should think really hard before sending more than 3. They won't look kindly on people who seem to have sent 5 just to send 5.