Only a pair will be tolerated in anything but the largest aquariums. They do well with other community fish such as clowns, firefish, blennies, dwarf angels, gobies, etc.
I bought 5 juveniles in the hope to be able to determine the sex of them as they got older. females will have one vent and males will have two vents (underside of their belly) -- as others have said if you don't have a pair (female and male) they will kill off each other until you only have 1