If that's the gist of your question, my first answer to you would be: "My assumption, until proven otherwise, is that he has enough ammunition to kill everyone in the place." Remember, with a pump or auto shotgun he can single feed rounds into that tube magazine and never have to open a cylinder like a revolver or change magazines like with an autoloader.
My second answer would be: "He's fired three rounds, that means there are three rounds left in his gun and he can kill three more people."
My third answer would be: WT* difference does that even make, and still what is your point?
I suppose on of my purposes is to expose jerks such as yourself.
Since there were 243 potential targets, anyone of whom might have been killed next, it is correct to say that each and every one of them was saved by the good guys!