I'm actually seeing two different questions here, even though it's phrased as one.
While home defense and CC could be satisfied with one handgun, I don't think it is. In your house (if you are home) you're most likely going to to end up in a safe room - not 8" concrete with lead lining walls safe room - but a designated room where there's only a single point of entry. Which means while you have made it so you only have to worry about one point of entry - you also only have one point of egress and they are coming in through it. Which means they are blocking your only escape.
What we were told in CC class is that after the first shot - even if it is the kill shot - the attacker can still keep coming at you for the next 30 seconds. 30 seconds. Sit there, and count out thirty seconds and see how long that actually is. And that's assuming they aren't hopped up on something. You really only want a handful of shots? Not me. Next to my bed sits a 15+1 M&P .40. If someone makes it all the way to my room, and is intent on coming in, it is obvious they are intending to do great bodily harm, at which I am unloading the entire magazine plus the one already chambered, and putting in the second magazine to make sure.
Unfortunately for carry, the thing is huge. I can carry it concealed in the winter under a coat, but spring, summer, and fall that's a no go. However, unlike putting yourself in a safe room, you are most likely going to have multiple/infinite points of exit even if you do have to fire. Which means, you most likely don't need 16 shots ready to go. Sure there may be a rare occasion you do. But that's a rare occasion inside of rare occasion, and there's no reason you couldn't carry an extra magazine on your weak side should you feel the need to. A extra magazine (or 3) plus my carry is still smaller than my full size .40.
So, for my carry I went with the Kahr PM40, and I love it. I actually shoot a tighter pattern with the PM40 at 10 yards than I do the M&P. Mine came with a 5 round magazine and an extended 6 round magazine. I generally carry with the 5. They also make it in a 9mm if you are intent on carrying a nine.