You can keep them in any number you like. I don't understand where people get these numbers...do the fish talk and tell us how many friends they want?
Yes it is true that schooling fish show more natural activity and are more enjoyable to watch in larger groups, but it is not a requirement for keeping them nor does it do anything to enhance their overall health or well being. Much less there is a minimum magic number.
Anyway, I have three so far and I plan to get a few more in my 55g tank. They're basically like corys in their needs but are larger, have more rays in their dorsal fin, and a more elongated snout.
I am half considering the purchase of a few aeneus corys just to see if they might school together.