Minecraft wiki entry for Bat, associated text: Bats can spawn in groups of 8‌[JE only]/2‌[BE only] in the Overworld at a light level of 3 or less on opaque blocks below layer 63. They also need at least 2 air blocks (or alternatively, a top trapdoor or top slab in the block above foot level). In Java Edition, between October 20 and November 3, they require a light level of 6 or less to spawn. On Superflat worlds, bats spawn below the top non-air layer, ignoring the usual layer-63 limitation.