You should get at least six clown killifish. Clown killifish are a social, schooling species that are best kept in small groups of even numbers, as they like to pair up within their groups. These fish have a generally peaceful disposition, yet male clown killies are known to become aggressive during their spawning time.
Killifish are best kept in small groups, and with tank mates that will not compete with them at feeding times. You can keep up to six clown killifish, which are also known as rocket killifish, in a 2-gallon tank.