I am asked this question often and before I give an answer, I always ask how often the person changes the water and cleans the tank. Getting the answer to this question is important for accurate feedback. Aquarium population and cleaning on a regular program will determine how many Discus fish can be safely added to an aquarium.
Discus fish belong to the freshwater South American cichlid family and therefore display a natural cichlid pecking order. I have found that adding more Discus fish to the tank calms their behavior by diluting this pecking order. If one cleans their tank and gravel once every 5 to 7 days on a regular basis, completes a 25% water change, and does not overfeed, you can stock a 100-gallon tank with 35 to 40 adult Discus fish. This stock level will result in your Discus fish growing evenly. There will be minimal pecking at feeding time and all the Discus fish will get their share of the meal. On the other hand, if one only wants to clean and change 25% of the tank water every month, then I would suggest following the one fish per 10-gallon rule and therefore stock with only 10 Discus fish. This stock level can lead to a severe pecking order resulting in uneven growth due to the dominant Discus fish getting all the food, growing faster, and eventually dominating the rest of the Discus fish in the tank. This is not good for your overall Discus fish community.
You can use the above formula to figure your comfort level and how much work you want to devote to aquarium maintenance and your particular tank size. For easy reference, if your setup is greater or less than 100-gallons you can do the math as follows.
150-gallon with once per week water changes Discus fish stock levels can be 55 to 60 adults, or once per month cleaning 15 adults.
55-gallons with once per week water changes Discus fish stock levels can be 15 to 18 adults, or once per month cleaning 5 to 6 adults.
30-gallons with weekly water changes Discus fish stock levels can be 6 to 7 adults, or once per month cleaning 2 to 3 adults.
The above formula is for Discus fish, any other varieties of small fish which are also in the tank do not count. Collectively the smaller fish will not put out a significant amount of waste.