Since the topic is "do you carry an extra magazine"...no, I don't.
The carry gun is a Glock 19 with +P ammo. That's a 17 round magazine and, as others have written, it'll be used as I'm trying to retreat. Being a civilian, I'm not going to go on the offensive if there is anyway to avoid it.
That said, there's an Ruger LCP (.32ACP) carried as well.
At home...if you knock on my door after dark, I'll have a 7 round Taurus Tracker .357 mag in my hand. There's also a 30 round magazine of 5.56 and an AR-15 close at hand.
If I'm home, I frequently have a snubby revolver in my pocket (I do right now, as I'm typing this).
I've found a 9mm is a great round for stopping an assailant or assailants. I have no intention of killing anyone, but if that's the result, so be it. There will be no hesitation on my part if I determine it's truly a threat.
One thing...with a carry pistol, don't trick them out or lighten the trigger pull. Any situation requiring pulling the gun will be stressful enough...you don't want an accidental discharge resulting from a tricked out trigger.
Also, my opinion...no need for a high priced firearm like a Les Baer, Wilson or similar. Odds are if you do use the gun, it'll be confiscated and you may not get it back. A standard Glock 19 may not be a tackdriver, but it's reliable and not that expensive to replace.