Some schools do one of those, and others even do both. It really just depends on the program. Smaller programs will typically hand out 1 acceptance at a time since they can't absorb extra students. Larger schools tend to accept a lot of students at once. They know that a certain number will not accept, and then they extend individual acceptances later. If they have a few extra one year, it's not a problem for those schools.
some of the program's sites tell you how many they accept, and how many they wish to matriculate. ie, u cinn says they send 12-16 acceptances for 6-8 seats. i'm not sure about the others, but i'm sure they also accept 1-2 at a time as dmblue said.