You should keep no more than a pair per 20 gallons at the least. Best to always give more room...after all...they are cichlids and can express the aggressions cichlids are notorious for. They are easier to deal with about aggression because they are on more of the peaceable side. A good rule of thumb to go by...introduce lesser aggressives first to avoid unwanted predeation and lessen territorial disputes.
Sexing discus accurately can only be done with adults during the spawning season. There is a difference in their lips however slight and takes good eyes for detail to distinguish the difference. I don't remember which is which, but one has lips that are even with each other and the other has a slight "overbite" (for lack of a better term). The lip looks a bit more puckered. The best way to insure a pair other than buying paired adults is to purchase juveniles and let them pair off. Even if you have just two juveniles, most likely one will mature into a male. It's a fishy thing. They don't need doctor appointments for sex changes. It just happens...
LOL.
It's a good thing not to just jump right into discus. As beautiful they are they are really sensitive and demanding fish in their care. They are perhaps the most difficult
FW fish to keep alive. Stay away from wild caught even though they do end up doing much better than captive bred for some odd reason, they do come in with hordes of parasites. Once treated, those that survive prove to be well kept and hardy in the captive environment. The captive bred animal, because of the parasite factor, is a better choice. Many are also bred in harder water and the offspring are more tolernt of pH values that would kill a wild caught. The easiest out of the captives are going to be the brown discus. Good species to start with before diving into the more brightly colored Diamonds, Pigeon Bloods, and Ghosts, etc.
Have a good size tank with good biological filtration. Discus do much better in planted tanks. Discus should only go into well established aquariums meaning...no discus till the tank has been up and running with fish for at least 6 months if not more. Before that time you can populate the tank with other critters the discus can live with. Cory catfish (the bottom feeder), otocinclus (the algae eaters), perhaps a school or two of tetras like cardinal tetras and/or harlequin rasboras to name a couple of examples. Do not add schools of small fish AFTER the discus are in there and well established in their territory. They will be eaten. They can tell the difference between fish that are in there before them and those in there after them. The discus should be the most aggressive in the tank community. They tend to do poorly if there's something more aggressive than themselves in the aquarium.
When dealing with any aquarium, but especially important when dealing with fish as sensitive as discus is to have a
QT. They should always go into a
QT for at least 2 weeks before being introduced into the main tank.
Read. Read. Read...and ask questions. Go slow. Time and patience is key to a successful system...not just keeping fish alive, but keeping them healthy and happy. Become familiar with the nitrogen cycle and upkeep for discus requires frequent, but small water changes. Usually 10% a week to 15% every two weeks. They do not handle large water changes very well. They also like a higher temp than most tropicals. 85 degrees seem to be favorable for them.