Depends on the size of the tank and the species of clown. If you have a decent sized tank and don't overdue it with tons of clowns then you should be fine. I would stick to a pair per 20 or 30 gallons or so of the same species. So if you want two different species i would aim for larger than 30 gallons, and if you want two pairs (4 total) then i would aim for maybe 50+.