30-40 clowns would be fine.
Here's a vid when there was 40+ clowns in a tank of similar size to your 180. Most of them were in the driftwood wall. Other vids are in my signature as well.
[video=youtube;YEEqaMJ-xZA]http://www.youtube.com/watch?v=YEEqaMJ-xZA[/video]