Six or more cory catfish should be kept together. Cory catfish are small fish and feel safer and happier living in a school of at least six.
Cory catfish will only form a school with other corys of the same species, so you should keep just one cory species in a tank.
You should also consider your tank size when deciding how many cory catfish to have. As a general rule, an inch-long fish requires around one gallon of water in the tank. Thus, depending on the cory species, you can keep between five and ten cory catfish in a 20-gallon tank.