Get 75g. It's much more spacious than a 55g. Start with no less than 4. If you do plan to keep juvenile discus (size no bigger than 4 inches), then keep the tank barebottom first for hygiene reasons. Juvenile discus require plenty of feedings which is compensated by several water changes to maintain good water quality. They are far less forgiving compared to adults and are easily stunted once the water conditions deteriorate quickly. Once they get big enough, then that's the point you may wish to start adding decorations there. Patience is a virtue that is highly needed in this situation. In the end, it pays off if you follow the guidelines set to help your discus grow well.