As far as capacity is concerned, that's a personal decision that no one can make for you. The number of rounds that may be more than enough for one person may be totally inadequate for another and vice versa. Because we do not know you, there is no possible way for us to tell you how many rounds is "enough" for your circumstances.
That being said, when I carry a semi-automatic I always carry a spare magazine regardless of capacity. This is because, in my experience, magazines are second only to ammunition as far as being the most frequent cause of a semi-automatic handgun jamming. Because of this, I feel it prudent to carry a spare magazine so that you can get your gun back into action should the magazine in it prove to be problematic. While I almost always carry at least one reload for any gun, I'd actually be more comfortable carrying a revolver with just the rounds in the gun than I would a semi-automatic.
That being said, when I carry a semi-automatic I always carry a spare magazine regardless of capacity. This is because, in my experience, magazines are second only to ammunition as far as being the most frequent cause of a semi-automatic handgun jamming. Because of this, I feel it prudent to carry a spare magazine so that you can get your gun back into action should the magazine in it prove to be problematic. While I almost always carry at least one reload for any gun, I'd actually be more comfortable carrying a revolver with just the rounds in the gun than I would a semi-automatic.