According to what I've read in Marine Fishes Pocket Guide by Scott W. Michael, "Can be kept in groups, but should not be overcrowded. Several juveniles can be kept in tanks as small as 30 gal, but an aggregation of adults should be housed in a tank no smaller than 55 gal. Ideally a group will be composed of one medium-sized or large individual and two or more smaller specimens. On average, males are larger than females; choosing one larger adult and several small ones will increase the changes of creating a reproductive unit. Will build nests and spawn in captivity." Hope this helps!
(My personal preference for my 55 gallon tank is having just one of several types of fish. I am limited by tank size, but this allows for a variety of "fish scape"..... )