This is a great question, I was wondering the same! I plan to register for the Interfolio site to manage my letters. (I'm sorry to use your thread OP however I have some additional questions and I figured it is related).
-Would we need to get an update to the letter if we don't get in the first application cycle? Are profs usually willing to do this (assuming they still remember you!). Just yesterday I had my summer chem prof offer to write me a letter and I didn't even ask 😀
-What information do we want to provide when asking for a letter? Should we have transcripts, personal statement, and resume ready?
-How many letters should we have: 2 professors, 1 MD, 1 DO, 1 employer? Or would I need more than this?