The rule of thumb I had alway heard was to hold no more than 2 acceptances at one time. Once you receive a second acceptance, then start deciding which of the two you do not want to go to and let them know so they can move to another person. Just what I was told.
You may be doing this, I obviously do not know. I would not think you would have to make a decision before April 15th, but it would probably be nice for those on the wait list if you do (wait list is no fun).