Magma cubes should spawn in groups of 1-4 or 2-5 according to the herd sizes given in the vanilla behavior pack spawn_rules\magma_cube.json. However, they only ever spawn one-at-a-time. According to this comment on MCPE-46540 that is because LavaSlime::checkSpawnRules() cancels spawns that would collide with existing entities, and in Bedrock Edition herds attempt to spawn all at the exact same spot (
- Create a large platform in a biome that spawns magma cubes.
- Run /kill @e 3 times and keep watching.
Magma cubes spawn in groups of 1-4 or 2-5.
Magma cubes always spawn alone.