Playing around with Passive Mobs in both Spawn and PotentialSpawn and found some interesting issues:

1. If I set passive creatures to spawn using PotentialSpawn - there is no passive mob cap so passive mobs are just created en masse (easily 300 in a couple of seconds). Maybe add a parameter to "followmobcap": true or "followpassivemobcap": true to the PotentialSpawn files?
2. A large part of Passive Mob spawning happens at World Generation, but the Spawn file does not give you control to add passive mobs and weights to be executed when spawning passive mobs at world generation.
3. I cannot get "MaxCount" to work in either files with my rules - it either blocks all spawning or nothing at all.
4. Command "/ctrlkill passive" also kills entities that have been spawned outside of passive (like ocelots that are spawned as part of regular mobs)

About 4. Of course. How would I be able to distinguish? Ocelots are passive mobs so they get killed

About 1. Potentialspawn has no way to enforce max spawn count. That is technically not possible. You have to combine that with spawn

About 2. That must be done with potentialspawn. Can't be done with spawn

About 3. Please show example

From memory, in Spawn file:
{
"mob": "animania:frog",
"maxcount":"16,animania:frog",
"result":"deny"
}

From PotentialSpawn:
{
"passive": true,
"biome": "Jungle",
"mobs": [
{
"mob":"animania:frog",
"weight":16,
"groupcountmin":4,
"groupcountmax":4
}
]
}

Result: I'm drowning in Frogs :-)

I will check it out when I am back from vacation

I think you actually want to use mincount in a deny rule like that, because you want it to deny the spawn when you reach the number, not when your less then the number. Really mincount and maxcount are misleading names, it's really more like countgreaterthen and countlessthen. Aka the rule applies in mincount if the count is greater then what you specified for mincount and less then what you specified for maxcount.

Still kind of weird that you can use mincount and maxcount in potentialspawn, and one of the given examples on the wiki uses it, even if apparently it doesn't work to limit spawns.

One last thing I noticed: I am still not at all sure if the counts are about the whole world or just the currently loaded bits or within the spawn radius or what. It's kind of vague. Obviously that is kind of an important distinction. I may not want 5000 of an entity crowding me in on all sides, but I don't really want like 20 to ever spawn ever. I think default minecraft just works by making it wait x ticks between spawns.

I am having the same problem as 1. I want to make a specific mob (turkeys) respawn somewhat frequently but reach a cap and not just accumulate rapidly as they do now. Is this possible to do with this mod? What exactly do you mean by combining that with spawns.json? I imagine add the potential spawns, then cap with spawns.json, but I see no cap option, only the maxcount option which the wording makes me think will count every single entity in the world.

The count indeed counts entities. Exactly like what a cap would do

Does this include entities not in loaded chunks? Because I suppose the game technically does already work like that, even if it's weird it can be avoided by not having passive mobs in spawn chunks, not keeping spawn chunks loaded, not putting passive mobs near chunk loaders etc.

But if it counts every single entity in the world that would be a problem for me, I need to have a cap on the amount of entities in the players normal spawning range not all entities that have ever spawned.

Can we please get some help with this issue?