To make it so not every horde you encounter after a few in-game days looks just about identical in number and distribution, maybe add three fields that govern rally group size and a percentage modifier that governs the likelyhood of that number being selected. Right now it's set at 20. Instead, for example you could have something like 4-8, 12-18, and 20-28 with 40%, 30%, 30% utilization respectively. Perhaps even spice it up even more by weighting the percentage rolls based on what size the "neighboring" hordes have selected to reduce the chance of one area getting too high a population. It would probably be beneficial to make a random-ish appearing adjustment for how the groups distribute themselves on the map as well, so you don't encounter those areas with neatly spaced zombie hordes dotting the landscape.