I know it depends directly on the number of mobs you kill. That said, I do not know of any method so far for controlling the number of mobs. I do know that stopping in certain areas for about a minute since the last mob pop as listed on this guide will always spawn an extra group of fixed size, but the sizes of various other pops throughout the run seem to vary randomly. Following the guide does guarantee 750/765 runs with enough time to register for the next run, however, if you have enough people.