One field per farm is enough; you can also harvest a field on several farms or harvest several fields on one farm one after another.
So we can have any number of each, but what is the best ratio of Farms to Wheatfields?
And from an in-game tip:
This means that you can have any number of farms to fields, but that the field will deplete faster if you have more farms around it.
So, given farms and fields are level 1:
1 farm + 1 wheatfield = t: time until the wheatfield is depleted.
Then increasing the farms gives us:
2 farms + 1 wheatfield = ½ t (i.e. the wheatfield will deplete at twice as fast)
4 farms + 1 wheatfield = ¼ t (i.e. the wheatfeild will deplete 4 times as fast)
Conversely, increasing the wheatfields gives us:
1 farms + 2 wheatfields = 2 t (i.e. the wheatfields will take twice as long to deplete)*
1 farms + 4 wheatfields = 4 t (i.e. the wheatfields will take 4 times as long to deplete)*
*note that as the farm will harvest one wheatfield at a time until the wheatfield is depleted, in practice the wheatfields are depleted one at a time (round-robin), in order of proximity to the farm, closest first.
So, what's the best ratio?
If you're trying to maximise production of wheat, then more farms harvesting fewer wheatfields is the way to go because it requires less resources to build and less space. However, this will require more micro-management to ensure the wheatfields are replaced when they become depleted.
The alternative is to have one wheatfield per farm, but level up the farm, then build the level 2 and level 3 wheatfields as they become available so you don't have to replace them as often.
Finally, if you place a number of farms and wheatfields together, then as the fields gets depleted the farms will harvest the neighbouring fields until they're all depleted or replaced.
Or your could use silos (which cost gems) to make it so you never have to rebuild your wheat fields. You would need the same number of silos as farms, or at least the equivalent total level of silo to farm ratio. This could be costly if you have a lot of production though.