After May 15 you can only hold one acceptance, but can stay on as many waitlists as you want. Take the first one that calls you. If you are accepted to a school that you like better than some of your other waitlists, remove yourself from them, but stay on any of schools you like better than where you are accepted. You only have to remove yourself from waitlists the day orientation or classes start. Before that, you can stay on as many as you like. If you are accepted off a waitlist after you are aleady in somewhere, you make a very quick decision, and turn one school down.