Minecraft wiki entry for Spawn, associated text: The number of mobs is checked once per each chunk against the cap. If the number of mobs (dead or alive) in a category is at its cap, the entire spawning cycle for that category is skipped. The area checked for mobs is the same as the area used for calculating the mobcap, which is the spawning area expanded by one chunk in every direction. The mobcap count is separate for each dimension.