Definately a social bunch and wouldn't recommend a group of less than three. To see them in their splendor, I would say no less than five. But that will depend on tank size and such.
Most of my tanks are small since they also double as invert tanks. I try to keep five in each tank. The only exception is the O. macrospilus. Only a group of three right now while working on them.
I had a lone female Vittatus sitting in one tank by herself and she didn't do much other than stay in spot generally. I finally moved in some juvies and she just sparked right up.
I've got a couple of videos that show how they are in big groups. Constantly on the move.