As a player I would like to set the minimum and maximum spawning pack size for a given mob.
spawnPackSize [mob] [min] [max]

Example
spawnPackSize witch 1 4

As noted in this video the mob spawning algorithm changed on 1.13, specifically how many mobs were attempted to spawn as a pack. This command can be used to buff witch farms, and nerf fish spawns. And it's eco-friendly because players don't have to run their witch farms for longer periods of time to make up for lost rates, saving time and electricity.

hmm might be fitting for carpet but seems like it goes quite deep in terms of setting too granular rules. Maybe that's something that could be added to extra, or to a separate extension.