If you don't mind returning fish, you could start with maybe 5 and then hope for a trio of a male and 2 females. You would probably end up getting rid of 1 or 2 fish that get ostracized from the colony but who knows. Male Multis get a little bigger than the females but other than that it is very difficult to sex them.
They will pair off and multiply rapidly. I don't like to advertise it, but my multi colony is in a 10 gallon. Though it's more crowded that I like, there's room for about three pairs plus tons of babies constantly. I would say 80% of the bottom is filled with shells.