I had a dream a few weeks ago where I was minding my own business in my house at night when someone appeared at my back patio door and was about to force their way in. My legs immediately turned to jello and I fell to the ground and couldn't get up or even move. I had to will myself to start crawling toward the bedroom where my handgun was, at which point I woke up. I realize it was just a dream, but it felt incredibly real, down to the clutter on our kitchen table. Since then, I have begun to heed the advice from MB about carrying whenever you can, wherever you can, including in your home. Also, now I'm a believer in another statement of his: if I get to choose what I have with me when facing a foe in a life or death encounter, I want to be able to call in an airstrike with a fighter plane to drop a 1000lb bomb on them or a helicopter with .50 cal machine guns, or an RPG, etc, etc. The point being, you want as much firepower as you can when you need it. Once I can actually find Hornady TAP again, my primary AR will be staying in the bedroom with at least 2 mags.
Back to the OP, I would agree with what has been said: as much as you can comfortably carry. For me, that is a mag of 13 rounds in the gun and another with 19 close by, either in a pocket when I can or in my EDC bag next to me when I am driving. I'd like to keep two more 19 round mags in the car and have a mag pouch on my belt as well, but that's not in the budget right now.