And don't forget that population = taxes = income. When your planets reach their population cap, you can add or improve a farm and the increased population will start pumping out more taxes. Just be careful, because more population means less happiness, which means you might have to lower taxes to improve morale, so it can be tricky and in some cases counterproductive to increase population. You can offset that by building or improving the morale buildings. My rule of thumb is one morale building per farm, but of course other morale factors (mainly the tax rate I can get away with, but there are others) often dictate otherwise.
I find generally that one farm per planet is enough for about planet quality 7 to 12, as long as the farm improvement research comes along when it needs to. Below that range, on the smaller planets I find there isn't enoug room for a farm and a morale building, AND other buildings such as star ports, factories, or research centers to make it useful for ship production, research, or in some cases influence generation. The exception is if there happens to be some bonus economy tiles or maybe bonus food tiles on such a small planet, where I will turn it into a specialized market economy planet that also has farms to support the population the markets will exploit (and the morale buildings necessary to keep it happy). Above that range of 7 to 12, there is probably room for two each of farm and morale buildings, or more if it's a really big planet (or again, if I decide to make it an economy specialized planet).
