You've likely heard this before.
It's the answer to a question you didn't ask.
It comes in a variety of flavors; salted with a drawl of some kind, topped with unbecomingly childish superiority, slathered with disdain, even tastefully prepared with the liberals' favorite spices: ignorance and confidence.
It doesn't matter how you season it. You can't hide the bitter taste of WRONG.
Aside from the fact that you don't know how you're going to be able to perform under the stress of a self defense situation (all you need is ignorance and confidence, then success is sure!), this statement has another problem.
Though I must apologize in advance for the problem I plan to outline. I know that most internet gun arguments use some degree of subjectivity, leading to an animating debate about the topic which eventually devolves into two parties picking up their jousts, climbing on their rocking hobby horses, and rocking endlessly at each other. Sadly, my point is not up for debate, because my point is math.
9 Attackers - 8 Rounds = 1 Attacker
You can only expect to shoot 8 attackers with 8 rounds. Any additional attackers are unlikely to wait patiently while you reload.
There have already been a number of reports of small mobs of blacks getting Justice for Trayvon® by beating white people (or people who they judge by the color of their skin to be white) indiscriminately. Race issues aside, when there are multiple reports of more than 8 people attacking innocent individuals completely randomly, you have to consider the possibility of facing more attackers than you have rounds in your mag(s!)
Pretty unlikely? No argument there. But you don't carry a gun because ever having to use it is "unlikely," you carry because you feel you have a duty to be prepared.
There might have been a semi-convincing argument if the comparison was between 8 rounds of .45 and 10 rounds of 9mm, but we're talking about over double the capacity in a comparably sized pistol. I don't know of any argument that can be made here with a straight face. In a worst case scenario, you can shoot the same amount of people twice and possibly have a couple rounds left over.
So when someone tells you that 8 rounds is enough, or some other flavor of the same, tell them you hope they never get attacked by a high-capacity mob.