It's a fair point to flip that argument. If the average civilian does not need more than 10 rounds, neither do the police.
What's that? The police DO need more than 10 rounds? Why? Whatever the reason is, it applies to the citizens they protect.
When I was getting my Criminal Justice degree the concept was supposed to be police going more towards "Civilians in uniform". Of course, that hasn't happened, they have just gotten more militarized. All the same, if a beat cop can carry a Glock 17 with 17 rounds in the magazine to protect himself from multiple assailants there is no good reason a citizen doesn't need the same protection. (I'll go ahead and take the penalty for the double negative).