I would do 4 max. The thing is, they need to be added all at the same time, same age, preferably from the same tank. If you only add say 2, but later decide you want 1 or 2 more, it usually ends up with a fight to the death. There will always be fights though. They will tear each others fins and sometimes you will find scars from a fight. It is amazing though how fast they heal. Mine actually just had babies again yesterday, too bad I didn't catch it in time and most became sucked into the powerhead or filter.