anyways npcs in villages based on number of doors do not breed-create more npcs, and npcs in villages after long periods of time will despawn.
i dont know if this is normal or what not.
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duplicates 1
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Adding to Eric's comment, you need 3 times more doors than villagers. For example, a village of 10 villagers would need 30 doors.
Also, there are now zombie sieges, were zombies can spawn on the village even if you light everything up.
Check the wiki, there is a lot of info there. Also, make some tests on a creative superflat world to get how the breeding works.
Nevertheless a duplicate of MC-1839
The breeding is based on the number of "valid" doors, which is calculated by a door with sunlight on one side and none on the other. A reason that the villagers are disappearing might be because of zombies, depending on difficulty. These may be bugs, but I don't have enough information to tell.