You can keep in any numbers as these are peacefull fish you could have 2 3 4 5 6 7 8 9 10 depending on size tank or sump or both lol
greatfish aswell. Best fish to start of with imo.
good for reef tank too.
Okay I have heard that they kill themselves off. What does that look like. I blamed my purple lobster for red patches that looked like bites on the side of mine. It looked like the scales were missing. They looked bloody. Is this what happened to yours? I like this fish because they school and look cool in a 125g. I started with 7 and took them back at 5. 2 died the rest were beat up.
I got 3 chromis in my QT right now to finish of my 75gal reef (had 4 but they killed of the smallest one) but from what I've read and my experience the really small one's don't fair to well so I recommend getting them in a medium size.
7 in a 180 should be fine depending on the rest of your future stock list and realistically you will end up with 5 since they will bicker amongst each other.