Hmm..
let me give it a shot... 😉
May 15th is when AMCAS WANTS you to choose one school that you have been accepted to. So if you get accepted off the waitlist then, you can hold on to as many schools as you want. May 15th is not the absolute deadline..usually schools will write you again making sure you are going to that school. By that point, the student needs to decide. So basically, you can hold to as many 'waitlisted' schools and one accepted school after may 15th. Schools are different at that point. Some schools will want a deposit usually..(not too much)...others will ask for another (usually a lot).
At that point when you have sent in your money, you should have also sent in a letter of intent. So let's say I got into UofPitt and I'm waiting on UCLA and UCI. Well, if I get into those schools (get off the waitlist) I can then decline my acceptance to UofPitt and go to UCLA. But the drawback is that I would lose whatever non-refundable deposit that I sent in. Geez, I'm writing to much. I hope this helps. Sorry if I went on a tandem. 😎