You can hold as many acceptances as you want until May 15, at which point you have to withdraw from everywhere but your top choice. Most is not all schools will refund your deposit when you withdraw before May 15. If you get in off a waitlist after May 15, you can switch to the new, higher-priority school and withdraw from the original. In other words, you can only hold one acceptane at a time after May 15.
This is all available in the MSAR and on the AAMC.org web site, if you want the official excruciating detail.