Minecraft wiki entry for Spawn, associated text: For each spawning cycle, attempts are made to spawn packs of mobs per each eligible chunk. An eligible chunk is determined by the same check for which chunks are random ticked. A random location in the chunk is chosen to be the center point of the pack. If the block in which a pack spawn occurs is an opaque full cube, further pack spawn attempts are canceled. There are a maximum of 3 pack spawn attempts per mob category.