Each professor will only need to submit theirs once. Interfolio will send each professor an email stating that you're requesting a LOR with instructions on how to upload it to the website. Once it's uploaded, you can have Interfolio send the letter to AMCAS. Once AMCAS has a letter uploaded, it will send the letter to each school you applied to on your primary. So make sure on your primary you designate who/where your letters are coming from so that AMCAS knows to expect them and subsequently send them to other schools.