Steps to reproduce

1. Load Animal Spawn Test World.mcworld. The player is standing 24 blocks above a 4 chunk x 4 chunk platform in a meadow biome. Only the platform is within spawn range of the player. Command blocks keep a count non-player mobs and rabbits. Above the player there is a repeat command block that can be toggle on to constantly kill all non-rabbit mobs.
2. Watch the platform and scoreboard for a few minutes.
3. Turn on the repeat command block above your head and continue watching the platform and scoreboard for a few more minutes.

Expected result

Up to at least 8 mobs can spawn on the platform before spawning stops, since that is the highest max herd size in any spawn_rules json file in the game. Sometimes rabbits spawn in groups of 5 or 6 since based on the spawn_rules\rabbit.json spawn giving them a max herd size of 6 in the meadow biome.

Observed result

Spawning stops when there are 4 animals on the platform. Rabbits never spawn in groups larger than 4. animals spawning in meadow.mp4