Mobs and Farms
Animals (i.e. chickens, sheep, etc) are limited to 25 per chunk, 250 visible in the same place.
Any farm that pushes mobs into a tight area must either be actively farmed, or ensure that mob cramming is in effect AND all drops are removed effectively.
Mob Cramming limits the number of mobs in a small area (entities take damage when they can push 24 other entities, including players). This does not affect drops though, so a mob farm that relies on cramming to kill the mobs but doesn't remove the drops can still be in violation.