In class, our instructor talked about a pecularity of black holes. When virtual particles come into existance for an instant at the event horizon, sometimes one is trapped by the black hole, while the other is able to escape. My question is, what determines the rate at which particles are created and ejected? Specifically, how come some particular black hole results in the creation of 10 particles per second, instead of 1000? It seems like "drawing something from nothing" in the case of particle/antiparticle pairs could happen at an infinite rate, so why does it settle at whatever rate it really is?