Minecraft wiki entry for Zombie, associated text: If a player is in a village with at least 10 beds and 20 villagers at midnight, up to 20 zombies may be spawned near the edge of the village in accordance with mob spawning rules. If a player is in a village with at least 20 beds and 40 villagers at midnight, up to 40 zombies may be spawned near the edge of the village. If a player is in a village with at least 30 beds and 60 villagers at midnight, up to 60 zombies may be spawned near the edge of the village, and so on. It can happen in any biome except the Mushroom Fields biome and its variants. Zombies spawned as a part of a siege are always normal zombies; zombie villagers, husks and drowned never spawn as a part of a siege.