I believe sunburst can be kept in a group of 3 (1 male + 2 females) I know a male/female pair will work
Some say that they do hang out in large groups in deeper water, but in shallower water, where most divers would be observing them, they are seen mainly as 1-3 fish. The species is generally more plentiful in deeper waters and the speculation is that in shallower water they may have trouble locating each other to hang out in larger groups.