A 75 gallon tank is perfect for a nice school of discus. I would say get around 8-10 discus for that sized tank. As long as you keep up on the water changes (1-2x a week would be enough, the more the better) and the temperature, they will do great in that sized tank. I would start off with smaller fish in the 3" range as they are cheaper and gives you a chance to see them growout over the course of a year or so. BUT, do your research before getting any. Ask questions. Join a discus specific online forum to learn as much as possible before making the leap into discus. They arent hard, but they just require a bit more work than most fish.
-Ryan