I have 6 juvenille discus in a 55 gallon currently and they are thriving and doing well. Unless you are water changing 70% once/twice a day or more you could definitely do more in that size tank. But if you live in the real world and have a job like myself and only do 50% every day or two I'd consider a 55 gallon the min size tank and consider 5 being the smallest amount of discus I'd have together.
So in my mind theres only 3 scenarios:
1) 55 gallon tank with 5 discus.
2) 29 gallon tall with a proven pair only. (plus a 75 to hold fry when they grow out)
3) 75/90(taller) gallon community tank with 5/6 discus + other discus friendly tank mates, bristlenose plecos, neon tetras, etc.