This is a difficult question to answer, and there is no single correct answer. It all depends on multiple factors, including:
How strong is your filter system? How often do you do water changes? Are your filters fully cycled?
Koi and Goldfish don’t mind being crowded, IF the water parameters are at the correct levels. We generally recommend two hundred gallons of holding space for one box of fish. However, they can be stocked at a higher density if the ammonia and pH levels remain consistent and healthy.