Murphy’s Law of General Magazine Usage:
Single assailant @ close range = One magazine or less.
Multiple assailants @ close range = Two magazines or less.
Running gun battles, or barricaded assailants = Three magazines, possibly more.
If you want to be, generally, well-prepared for a gunfight: Carry two pistols, one primary and one secondary, and four extra magazines, two for each gun. And, oh yeah, remember to wear your vest!
Of course, Jeff is right, all of this means nothing if you can't shoot straight, handle your weapon clumsily, or worse, lack nerve! Of all the abilities I've practiced for many years to achieve, the one talent I cherish most is the ability to become very angry, very fast! Without this, everything else can turn into a pile of, 'slow motion crap' right before your eyes!