tank size?

[FSM]

You could maybe have 8, but just having 6 in a 6X2 would give them more room

6 in a 180 if you keep them for 10 years. They aren't like cichlids; they grow ridiculously slow. ewurm has about 40 in a 75. He's got a blog post about it which you would probably find useful.

Unless you get big ones to begin, a 75 is a good start.