It's also a matter of ammo selection. I keep a mag of fmj and a mag of JHP, depending on if I need to fire through walls,"interior" the exterior walls are concrete. So nothing is getting out. But if I have BG's outside my bedroom door, I could easily put several rounds through the plasterboard walls, or door. as no one else is in the house. Same with cars. If someone pulled up to your home, and shot at you from a vechicle, you have the option of switching to a FMJ round. I need to put that foil on my windows, it will slow down if not stop a 9mm round. I have been looking at some different companys that make the stuff. It's expensive, but less than bullett proof glass.
But most times after a shot or two are fired, everyone runs out the nearest exit. So having them is more mental than actually necessary. If you have 15 round mags, 2 should be more than enough. 1911's may require 3 or 4, if the fight continues outside.