Question: how many doors do you need for villagers to spawn?
Answer: Each door can support . 35 of a villager, i.e. you need about 3 valid doors for each villager (more precisely, every 20 doors support 7 villagers). The boundary condition seems a bit hazy, but the 16 block requirement in the first point seems to imply that it's 16 blocks from the most outlying door.