That’s a great question, and there are some important considerations to remember when you’re calculating your fish load. The first is to remember that koi will grow. If you’re starting with young koi in the 6” range and you fill your pond to the max, the koi will quickly begin to suffer. The water quality will decline as fish waste increases and it may be unable to hold sufficient oxygen to keep your koi from gasping at the surface. The kois’ growth will become stunted and they may eventually die. When you’re starting off a pond, plan your load for their mature size and you won’t be faced with thinning out your koi in the next few months. You’ll also need to have a well set up pond for the healthiest koi possible. A depth of 4 or 5 feet would be ideal, to allow room for your koi to rest when the weather is particularly hot or cold. Koi tend to uproot plants, so you need to make arrangements to manage the level of free nutrients, or you’ll get algae blooms and water chemistry imbalances. One option for including plants in the mix to take up nutrients would be to essentially wall off part of the pond and keep a healthy variety of aquatic and marginal plants behind it. Adequate aeration and water circulation are critical considerations as well.
If you’ve got a well-set up, stable pond, a good rule of thumb is 1” of fish for every 10 gallons of water. So for an immaculate 500 gallon pond, you can safely accommodate no more than 50” of fish. That sounds like a lot, but accounting for an approximate mature koi size of 12”, that allows only 4 fish. That doesn’t mean 4 or 5 fish. It means NO MORE THAN 4 fish. In fact, 2 or 3 would be safer. If you were hoping for lots and lots of bright, colorful fish, a good crop of 30 or so shubunkin goldfish might be a good compromise until you’re ready to upgrade to a larger pond.