In my save, I discovered that it's not necessary that the working station is inside the territory of npc as long as it is near near them.
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NPCs will check for job sites within 10m (4 platforms) so long as their territory circle is set to anything bigger than the smallest value. So yeah, the job site doesn't necessarily have to be within the circle, though it's often a good idea to do it anyway.
Still isn't doing anything (Garlic was planted by me)
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DO NOT let a single NPC tend more than 4 plants by herself.
The farming AI is stupid and will randomly pick plants to tend to (or EMPTY PLOTS *facepalm*) instead of harvesting ripe crops. And if a ripe goes neglected too long it will wilt and when the NPC gets to tending it she will just replant the crop without getting the extra seeds.
Having them tend 8 plants increases the chances of them doing stupid shit and wasting their efforts.
Putting seeds in the harvest box is pointless. NPC don't need seeds to replant. They just replace a Stage 3 or Stage 4 crop directly with a Stage 1 without using any seeds. You get a crop (teleported straight into the harvest box) if it's Stage 3; you get A WHOLE BAG OF NOTHING if the idiot NPC picked a Stage 4.
So... assign one NPC for every 4 plants (one worker per wooden planter, two workers per stone planter). Make sure there's a well or water tank nearby and the NPC's territory circles are set to anything above the minimum. ALSO MAKE SURE very slot in the planter is filled. There are ways to optimize this further (involving limiting NPCs to specific planters) but trust me; it ain't worth the effort.
I tried.
Edit: Oh and one more important thing: farms only work when you're close enough to them. Go too far away and the whole map chunk with the planters will unload and the NPCs there won't do any work.