The chances of becoming involved in a lengthy, drawn-out, TV-style gunfight are incredibly low, be you a citizen or a peace officer. Fights are practically always over after the first gun full has been fired. Either the bad guy or the good guy has been hit, has given up or has run away.
The first shot is the most important. Experience has shown us that, if we haven't concluded matters with the cartridges already in the gun, firing another 5, 6 or 15 likely won't change things.
Off duty, I carry a Colt Commander and one spare magazine. I have found 7 round magazines to be slightly more reliable than anyone's 8 round magazines, so I have 15 rounds on me.
If the weather is really warm, I may leave the Colt home and carry a S&W Model 642 with a speed strip of 6 rounds in a pocket. I used to carry a speed loader in a jacket pocket if it was jacket weather but now usually carry the Colt if I need a jacket.
I keep a spare magazine or two and a speedloader or two in my car and my pickup truck.
If I have a second gun on me, it will usually be a Seecamp .32. (I'd like to find a Seecamp .380.) No spare mags for it. As the designer says in the little gun's instruction manual, if you think you need a spare magazine for a Seecamp, you really need another kind of gun instead.