Five, plus or minus one. Four is pretty much a minimum for medical schools. Six is pretty much a maximum. And if it does add something, get rid of one of the other ones. Two should be from scientific fields.
Students often have older recommendation letters from freshmen and sophomore years in their folders.
These may or may not be used for medical school at your discretion. It is nice if all letters are specifically for medical school and not generic letters for any old purpose. The people who you are probably planning to approach have been through it all before and will see it all again. But you should still perform all the correct steps. Meet with the person, either by making an appointment or during their office hours.