depends on a couple of things,type of clowns,size of tank and other fish in tank.I have a friend who has a 200g(uk gallon) tank with 4 different pairs of clowns which get along.It also depends on age of clowns.
Clowns which are young(around 3-6 months) get along fine and will do so if in large enough tank with enough territory spaces and with variuos fish.I have at the moment 50 young clowns in 2 very small tanks which all get along fine,I also have another tank with 2 perculas(paired and breeding) and 2 picasso clowns just pairing which all get along fine.Please dont quote me on this but clowns can get along fine but i reccommend they are very young when brought together,otherwise clowns can and will be very aggressive towards each other for dominance and territory.As with their personality which can be total amazing to each other so can their dominance and aggression.No two clowns are the same