It's plenty of ammo unless it isn't. The gunfight lasts as long ss it lasts. Your marksmanship, their marksmanship, what they are wearing, any substances they are on, how determined to finish the fight they are, their will to live, their desire to live, their mental health, how many there are, etc... ALL come into play. The other guy(s) have a say in it. In reality as they are the aggressor, they have more say in it than you do.
Most defensive shootings i have investigated have had fewer than 10 rounds fired. Most, but not all. I recently posted a story in another forum where one guy fired 15 rounds from his handgun and 30 rounds from his rifle in a home invasion scenario.
I wouldn't feel undergunned with a 20 round mag, but i only use them for shooting from a bench. I buy and use 30s.