I agree that a lawyer could get you around it with an explanation but the odds seem more unlikely if you are carrying more rounds. I could be wrong, I've never been and hope to never be in a situation like this. The reason I say the odds are unfavorable is because the prosecutor may try twisting your motives around, you know how they can do that.. Unless you have a VERY good lawyer, it's probably going to be a bad experience for you. I'd rather save myself the time/PITA in court if I had to appear.
I don't have statistics but I would venture to say at least 80% of SD cases that no more than 10 rounds are fired by the person practicing SD, so the 16 that my gun holds should be plenty. I don't want the prosecutor twisting things around acting like I'm a gun toting fool who carries ENTIRELY TOO MUCH AMMO for a regular SD encounter. (As stated before, extreme circumstances are exceptions) I'm not disagreeing with any of you who do carry extra mags, but I just don't feel the need to myself. It's all a matter of opinion I suppose.
It's hard to say you're carrying too much, if you're carrying less than a police officer. They fight the same bad guys we do. While they are more likely to bump into the need for their gun, once it is needed there is the same damage needed.
And if 80 percent requires less than 10 rounds, it follows that 20 percent need MORE than 10 rounds.