Just curious what other people do?
Is there any benefit to allocating 16 workers to a 12 x 12 farm, or can a lot less do the same job?
I always use the fault number when you set the field up. Iirc for 12x12 that’s 8 farmers. You see the number of farmers as you lay down the field; the bigger you make it the more it needs. You can try doing with less, but then some planting/harvesting might not get done.
I have to use 12 to 14 on wheat or there’s always mass crop loss 
for a 12 by 12.
My pet hate is farmers running about all over the place letting crops rot, because I’ve placed one one small farm on the other side of the town, it forces the player to build all the farms in one place 
One field 8x8 (50% fertility) requires at least 1 farmer.
The more fields you have the more extra farmers you will need because of logistic expenses - farmers would waste some time moving from one field to another. So I usually assign 5 farmers on one 15x15 field (fertility 50-60%).
Fertility factor and crops type affect that as well - field with higher fertility or certain crops (like wheat) produce more food units which requires more units of work to gather so you need more farmers per field.
You can experiment starting with low number of farmers and if you see delayed harvest and lost crops to rot then just add extra farmer to that field.