With respect to LEOs and capacity limits, there are two important concepts that really ought to be kept separate. One is: What are the odds of needing a firearm for defensive purposes? The second is: How many rounds does one need, in the event that one does need a firearm for defensive purposes. Those two issues often get tangled up in the discussions about LEOs and capacity limits.
Due to the nature of their work, LEOs have a higher probability of needing to use a firearm. I, as a civilian, do not do things like make traffic stops or strike up conversations with people who are unruly. LEOs do. That means that it is more likely that a LEO will need his gun to defend himself. By extension, this also means that LEOs face an increased probability of multiple attackers, as compared to myself.
However, once that "probability threshhold" has been crossed, and it's clear that a defensive gun use is in play, a law enforcement officer faces a human or humans, just as you or I would. Further, LEOs have the advantages of being able to get backup, and (normally) having notified someone of their whereabouts before making contact.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.