Iron golems spawning behavior it's completely different in comparison to hostile mobs.
Iron Golem Spawning
VillagesIron golems will spawn in a 16×6×16 area, centered vertically and horizontally between the bases of the 21 or more valid doors in a village if it has at least 10 villagers. The chance of spawning is 1 in 7000 per game tick, which averages around one every six minutes. Iron golems can spawn provided the blocks it spawns in are transparent and the block it spawns on top of has a solid surface. The maximum number of golems that can be in a single village is one-tenth the number of villagers in that village, provided that the door requirement is met.
There are many iron golem farms out there that you might want to check out.