I'd just say to have that spare mag ready when you see the number. If your serious about defense, you should be at the least a common sense tactician. If you don't have enough to do two in the chest and one in the head per person at the least, you should have extra rounds ready for the occasion. But that is a difficult scenario. I'd have to know what the layout of the house was and know where they broke in at. I teach this stuff so I'm constantly thinking about this. Just as no two shots are the same, no two criminals are the same
Not much would change for me accept if there were more than 4 targets (Bad Guys).