9 doors wont make them breed in my villager breeding area i have 16 houses with 4 doors on each also is the roof glass if so villager wont class it as a house
place 2 villagers on the floor, then 10 block above the floor do a 5x5 platform, fill it with doors, do a roof on top of them and add walls. and a villager inside.
If at least the definition of a valid door on the MC wiki is correct, you should be able to create an arbitrarily long, 8 blocks wide area, completely filled with doors with solid blocks on top, and each door should count towards the population.