How many blue chromis should I get?

[Patty-Brent]

Hi I currently have a 55 gallon long with a yellow tang Midas blenny a spotted mandarine and 2 ocellaris clowns. I have around 50 pounds of live rock and Many corals. I'm just wondering how many Blue chromis I can get? I feel I am missing some blue fish in my tank... Any advice would be appreciated The tank has been up and running for over a year... Thanks in advance!