Yes discus are schooling type fish. They feel safer in larger numbers. If you keep one, it'll feel less safe and become stressed. If you keep more than one but less than six, they will pick on the runt a lot, until they either kill it, or it dies from stress and finally some disease.
Discus need at least 10 gallons per fish. So really if you want to keep discus, you need at least a 60 gallon tank to even be able to play. That's just the discus, no other fish. If you want other fish in there too, and there's really only a few that work out with discus anyway, you'll need an even bigger tank.