Minecraft wiki entry for Spawner, associated text: A spawner activates when a player comes within a spherical radius of 16 blocks from the center point of the block; i.e. 15.5 blocks from the spawner itself. In Java Edition an active spawner attempts to spawn mobs within a 4-block horizontal and 1-block vertical range; that is, in a 9×3×9 volume centered on the spawner. In Bedrock Edition, the horizontal spawning range is 4 blocks taxicab distance, creating spawning volume extending 4 blocks in each cardinal horizontal direction from the sides of the spawner; its horizontal cross-section is therefore diamond-shaped. Mobs can spawn anywhere in this range that is suitable, with mobs more likely to spawn closer to the spawner than farther away.