I believe that 10 is a good number, I have had mine for over a yr now and they aren't aggressive toward one another at all. My juv emepror don't like them much though. Just a disclaimer I started with 25 of them and now have 10, not because they killed each other but during the first month or so of having them they slowly died off for no outward reason ie disease. Most people get them because they school, which mine still do but after awhile when they become comfortable in your DT that will stop too