500
During this situation you would would response with your M4 at Port Arms a round in the chamber, safety selector on "SAFE" and the finder not the trigger guard and the M9 carried in it's holster, flap open/sentry lock disengaged and hood rotated forward, shooting hand on the pistol grip, hammer not cocked, and finder not in the trigger guard.
What is responding to and ACTUAL incident where you can reasonably expect to meet and armed adversary (203)