It depends on what type of food is being produced, since time increases for different plants.

Wheat is growing the fastest, so I assume you just need 4-5 fields. You can create a common space for efficiency, Farms around farm fields, then you will have farmers always picking something up.

If you grow sunflowers, you may get way more farm fields.

Trees require 2x2, so you also need a specific setup and more room to plant.

Although my blueprint for a farm doesn't include a standard field, I find a 4x% or 5X4 field size works for anything I'm growing..

I've noticed that if I start with only a few plants or animals, either sized field fills up quickly.

Make sure your plant or animal limit corresponds to your field size.

my farm is not working!! The wheat is grey and cannot select and plant to grow.

wheat must first be found on the planetoid and collected

For wheat it's something between 4 and 5 farm plots.