This depends on the size of your tank and the fish you want to grow, each pound of fully grown fish requries around 8 gallons of water. However, this number may vary depending on the species of fish you're raising. Bigger and greater numbers of plants will require more nutrients, and therefore more fish waste. This will either mean choosing particularly wasteful fish, or increasing how many live in your tank at a time.
If you're planning on growing a lot of plants, or harvesting a lot of fish, you'll want to pack your tanks a little tighter, as long as the chemistry remains balanced. A high-production system may want to only allow 1lb of fish weight for every 5-7 gallons of water. For example, if you had a 500 gallon tank, and your fish were in the 2lb range, you would need to keep your population below 250 fish. An easy way to do that is by harvesting the big ones and replacing them with fingerlings who can then use the extra room to grow! If fish production is not your focus and you’re planning on keeping the minimum to support your plants, then you can go the other direction and calculate that you need approximately 1lb of fish for every square foot of grow bed surface area. For example, if you’re planning on setting up 3 beds with a surface area of 2’ x 4’ , that’s 24 feet of grow area, so ideally you’ll have about 24 lbs of fish. This can be done through few large fish, or many small ones, depending on your needs and plans.
