How many letters are you allowed to submit? Also, can I have different letters go to different colleges? i.e. If I have four letters, can I have 3 go to one school and a different 3 go to another?
Four is not excessive at all. My school's health professions committee has us submit five for our file that they then send to schools. I'm not positive, but I don't think there's an option to submit certain letters to different schools. Regardless, I don't know why you'd want to hold back. If they're good recs, they'll only help you.
Yes you can submit different letters to different schools! Many schools have different letter requirements. For example, some want just 2, some want a non-science, some want 2 science, etc.
You can submit up to 10 letters through AMCAS, but I think giving 10 to every school would be excessive but four should be fine. Also, note that a committee packet only counts as one letter even though it might actually contain letters from a few different professors.
Also, yes you can submit specific letters to specific schools if you had a situation where a specific letter of recommendation you only felt pertained to one school.