I have 4 together in a very very large house. at one time, they had to be kept in a 20 long for a few weeks. You know what? no one got eaten or hurt or sick while in the 20 long! I keep mine well fed with many hiding spots. Even though there are many hiding spots mine like to hug each other in the same cave. While in the 20 long I also kept them in obervation to see what sort of aggression they would display. I quickly mapped out their behavior when it came to interacting with others they come across and agression seen during feeding. No one in my family appeared to display violent behavior.
so a good rule is, you can keep a whole large happy axolotl family. The secret is, keep well fed, plenty of hiding spots, and always observe to see if one starts to become a bully. A good and healthy axolotl house should have no problem keeping an axolotl family nice and safe. If you neglect their care they may act out, so keep up with care.