The number of shots taken matters only to the likes of Jessee Jackson and Al Sharpton, the race hustlers.
Once a policeman (or anyone else for that matter) decides to use his weapon he fires until the assailant goes down. He is taught to shoot until the threat is stopped. The number of shots is irrelevant to anything else. How ever many is takes is simply how many are needed to stop the threat. This is also true of civilians who know anything valid about self defense.
No, it's neither difficult or easy to take less shots. Typically, he will fire two (called a double tap) and assess, then fire more as needed. I'll tell you this, if what we've heard is true, that Brown attacked him in the car and tried to take his weapon, I'd fire at that charging 300 # man until the magazine was empty and then reload for a second string if that's what it took to stop the assailant.
Determining if someone is armed might be difficult if they don't show the weapon. If they don't show it, it would be difficult for them to use it on you, wouldn't it? However, one does not have to be armed to be a deadly threat.
I've answered based on your questions, not based on the assumption that what we do know is all true. We still don't know all of the facts and my answers don't presume that we do...