The number of discus in any tank is totally dependant upon your filtration and number of water changes. The more biological filtration you have, the better. Using a wet/dry filter is ideal for any discus tank as it holds a TON of biological capacity. Also, the number of water changes...the more you do the better, especially with a heavily stocked tank. If you wanted to, you could house 50 3" discus in that size tank...if you do 75% water changes 2x a day and use a big wet/dry filter...but realistically, aim for a stocking density of 1" per gallon...If you get 3" fish, you can house 12-15 of them in your size tank. Or if you want to be even more conservative, go with the standard 1 fish per 10 gallon rule. The more water changes you are able to do on a discus tank, the better.
-Ryan