As others have pointed out, you can hold as many acceptances as you want until May 15th at which point you have to pick one school. If after may 15th you get another acceptance off a waitlist, you have another certain time period until which you have to determine again whether to keep the school you chose or go with the school you just got off of waitlist from, etc. And that is how it generally goes.
Don't give up any of your acceptances til you see what financial aid packets they offer, and you've thought about where you would rather go really long and hard.