6, 2M 4F or 3M 3F. I kept 3M 3F in a 60x15" footprint, had some minor aggression issues, but all in all it was a pretty comfortable setup. I may have been lucky though.
Just keeping 4, with 2 pairs, would also work nicely.
If they're young you can have more, then thin the herd later. I'm a big overstocker though. Mine don't grow super fast so I've crammed a whole bunch into different sized tanks while they were in the 3" range.