Here is a good link on this topic. JohnKSa's "Capacity, Hit Rate and Multiple Assailants" thread. If you shoot as well as a typical big city police officer in LA or NYC and manage a 30% hit rate, and you face the typical scenario (2 assailants), a 10 round magazine gives you a 35% chance of being able to land 2 hits on each attacker.
When you start thinking about how likely it is you get effective hits, you start getting an appreciation for just how much of a handicap people who suggest magazine capacity limitations are really pushing. And worth noting, just a measly 2 extra rounds (12rd magazine) brings you from a 1/3 chance to an even 50% chance of getting 2 hits on each attacker.