Most medical schools require a minimum of two letters from science professors and 1 letter from a non-science professor. If your school has an undergraduate premedical committee, it is best to have them write a letter for you as well. Although the minimum requirement is usually 3 letters (if your school does not have a premedical committee), generally it is best to have 4-8 letters. Also keep in mind that these are general rules. Be sure to check each school’s individual letter requirements.