When configuring custom spawners, the "MaximumNearbyEntity" tag which is intended to set a cap for the spawner in relation only appears to have a vertical range of 4 above the spawner and 4 below the spawner. The Wiki states that the y range should be '"SpawnRange" +1 centred around the spawner block'. However, setting the "SpawnRange" tag to anything will still result in a above and below value of 4. This also means that if the position data of the spawned entities is above or below the vertical limits, the cap won't work correctly and they will continue to spawn until the game crashes. I am unsure if this is a bug however it seems desperately lacking as a feature if not in that the horizontal cap range can be made to be much larger than this.