Valid question, as most civilian gun fights are under 10 rounds according to our scientific research (aka, Google-Fu). So a magazine with a capacity of 19 rounds should be more than enough.
But that’s not the most important factor to consider.
What is the most important reason? Magazines fail, it’s just how it is – and Murphy’s Law can come into play when you need your firearm the most. Broken feed ramps, worn springs, dirt/crud in the magazine (possibly during a scuffle, etc). Magazine issues are one of the most common reasons for handgun failure, why rely on just one when it’s so easy to carry a backup?
If you don’t carry a spare magazine, carry a spare gun (aka New York Reload).