1-2 Spares
Generally I have 1-2 spare mags on me. Not that its likely I need that many rounds, but for a variety of reasons I think its prudent.
First off, you don't know what the situation you might get caught in could be that forces you to defend yourself. It could be a riot, in which case you may wind up having to face multiple successive events where you are having to present or shoot to save your own ass. Having seen the LA Riots first hand, having had to go to work during the course of the riots, right in the middle of the affected area I can say with certainty that this is a possibility that looms before anyone in an urban area due to natural disaster or manmade foolishness.
Second, you don't know what the situation you might be faced with is. One attacker or several, in cover or in your face. I carry the mag & gun loaded with Glaser Silvertips, since its most likely (by probability) that I'd be facing a single attacker, in direct line of sight. Glasers minimize the over-penetration issue or missed rounds passing into structures where they could harm others. They aren't reputed to be very comfortable for the bad guy who gets hit with them either. My second magazine is loaded with JHPs - Corbon, Critical Duty, Black Talon-esque expanders, Buffalo Bore etc. These are expanding bullets, but they have a much higher penetration potential than a pre-fragmented round. If I am faced with a situation where I am forced to defend myself and wind up fighting someone in cover, I can swap mags for the heavier, more solid rounds. Chances are if I emptied the first mag its going to be a cover fight at that point if it is still going. If I have a third (depends on the rig I have on) its usually something like Critical Defense or Mag-Safe ammo, its more of a backup of the first with limited penetration ammo.
Third, someone mentioned magazine failure. This is a critical thing - if something can go wrong, it will do so at the most inconvenient time. A backup is prudent doctrine.
The carrying of extras also depends somewhat on the capacity of the firearm. A high capacity (15+) firearm is worth a full mag and a reload on a 1911 style firearm. Plenty of people say stuff about not needing that many, only one shot etc. That is macho chest thumping, when TSHTF its said to be true for most people that they will shoot like their worst day at the range with the adrenalin shakes & all.
But hey, look at NYPD - their finest can only hit 9 bystanders in 16 shots... Wonder how many it was hit the guy they were aiming at?
