Minecraft wiki entry for Breeding, associated text: A player may want to know the number of mobs required in a farm to reach a certain goal, for example: to make a full-powered enchanting table with bookshelves (46 leather needed) or for full leather armor (24 leather), in case the player has a cow farm. There is a formula to calculate how many mobs are needed in a farm, by the starting number and if the player waits until all the mobs become mature (this does not apply to villagers):