I would go with three, one male and two females. Though my males tend not to nip each other in this tank , when they were in a smaller one they occasionally had little territory discrepancies, none of them which resulted in any injury. You could probably push it to four total gudgeons, but three would give them ample space. They like plenty of vegetation and caves and tunnels. Mine have PVC pipes they use for breeding as well as a plant they are all obsessed with (even though it is a fully planted tank). They are a very peaceful species and aren't very active like tetras, but they have beautiful breeding behavior and their colors are fantastic.