Minecraft wiki entry for Spawn, associated text: When the max pack size is less than the number of possible spawn attempts, some spawns attempts fail, but are seen more commonly in practice. Based on the number of mobs that have been successfully spawned. If the max pack size is greater than the number of spawn attempts, one gets only the number of spawns from the spawn attempts. Some mobs have a minimum and max pack size, meaning there is an even chance for any number of spawn attempts between them occurring.