It varies from program to program. Three letters are most common, some will ask for two. Majority will ask for dean's letter though. Others will be form US professional and someone who observed you in a clinical setting back home, for example - previous employer or professor. But again - you have to check each program. One may have specific requirements, other will just ask to provide letters from someone who observed you, no matter if it's dean, US or foreign doctor.
A lot of people agree that letters from US doctors have more value than those from your home country.
