Breeders in Asia throw the 10 Gallon Per discus rule of thumb out the window. They often pack their discus in there and they are all healthy!! But the secret is their heavy water change schedule. They do 100% WCs at least three times a day. Of course that is an extreme case but the worse thing you can do is over crowd discus and neglect your water changes. Once a week will not cut it. You can get away with a higher count if you keep a bare bottom tank and do frequent water changes.
If possible, your water should be aged, pre-treated, heated and aerated instead of water straight from tap.