Villagers and iron golems seems to count towards the passive mob cap. If I have a bunch of villagers or iron golems, no animals will spawn in the world.

Steps to re-produce:

• Create a flat glass world with plains biome
• Create a platform of solid spawnable blocks, watch animals spawn on the platform
• Spawn a bunch of villagers/iron golems/both nearby (I spawned 30 in 30 cells)
• Kill of all the animals, and watch the platform. No animals will spawn any more.
• Name tag all the villagers/iron golems, animals will start spawning again.

This is a major issue, and will affect passive mob spawning a lot.
If you have a villager breeder in you spawn chunks, or a village actually spawn in your spawn chunks, there will be a lot less animals spawning in the world.
And for multiplayer servers, it's enough for one player to have a villager trading mall, and it will stop all passive mob spawning for the entire server.