I have a main disturbution panel that feeds a panel in a new addition. The panel is a 3phase 480v and has 12 circuit breakers in it. My understanding is that since there is more than 6 it has to have a main breakers or switch to kill power in case of emergency. Since it have a main disconnect in the disturbution panel does it need one at the panel? The two panels are about 400ft apart.

As long as the sub-panel is in the same building as the MDP it does not need a main breaker. This is a very typical installation.