It depends on the species of clown. Black ocellaris can live in pairs for sure, although I'm not sure how well they tolerate more than 2 or if they tolerate other species. Orange and white ocellaris do well in groups -- if you put more than 2 in the tank, they will form a group where one will be the largest, dominant female, then below her will be a submissive male, then the rest in the group will be neuter/genderless. If the dominant female dies, the submissive male will turn female and take her place, then the next-ranking neuter fish in the group will turn male and so forth. Maroon clowns, however, do NOT tolerate any other clowns in the tank, and it can be hard to pair them up.
As a side note, I would add more live rock. Generally 1 pound per gallon is the goal if you want proper filtration and hiding places for the fish. Good luck!