It's rare that one can be "overgunned". The worst case scenario is using your firearm, and with the threat still present, and being out of ammunition. Of course, the argument at that point, is you fired 6-10 shots, and the attacker is still present, then where exactly did your rounds go? Suffice to say, I'd still error on the double stack magazine, with 1 spare. In case the attacker has friends that aren't so easily dissuaded