They're social animals, and prefer numbers for security and probably companionship. In the wild, they'll break into groups of 10-12 to forage, but keep an eye on each other. At night, the larger groups can reform. They can also travel in huge packs of hundreds or thousands at times.
So you have a social animal. Because of tank size, we fishkeepers pulled out the number six. It's arbitrary. Some research suggests they do best with at least 10, but again, the principle is that they are social. Bear in mind that they can live 15 years, and you have lots of time to build the group up as the lfs brings in orders. It's a popular species.
I bought a group of 6 of an undescribed Hoplisoma (cory) and had four die after shipping. I held on to the 2, who were rarely ever seen in their tank. When they came in again, a year later, I bought 6 more (budget decided the numbers) and combined them after quarantine. The two hiders are now front and centre, all over the tank with their slightly smaller gang following them. The behaviour change was radical, and overnight.
I'm calling happier, as they don't hide or vanish for days on end. I have had the same experience with concolor and melini, both fish that also came in with losses and showed up again a year later. I have 9 concolor and 11 melini, and they are delight to watch.
