Like mentioned 3 is a good number and that number depends on the number of water changes you are willing to do, the filter, decor and size of the fish. I have heard that some people have 10 discus in a 30G but they also do water changes every second day. In my 90G, I have 6 discus and I feel that I could put more in. Everytime I do water changes(twice a week), I also test the water parameters. The no3 never goes up not even if I miss a water change. I will wait and see for another 2 months though before adding anything.