The number of discus in any size tank is 100% totally dependant upon your water changes and filtration. A good biological filter is key to a healthy discus tank...large and frequent water changes is an even bigger key. In your signature, I count 11 discus in the 80 gallon. Thats number is just fine. About average stocking I would say for most people anyways. You could keep say 50 discus in the same size tank, but you would have to increase your water changes to a daily basis and very large (75%+) and increase your biological filtration significantly. But for your tank, 11 discus is just fine, even when they get to full size.
-Ryan