I forgot to mention in my initial post (and somewhat of a sidebar) that I agree that two spare rifle mags are probably adequate for 99% of situations. However, officers should consider their individual or department circumstances as well. For example, in the North Hollywood incident, LAPD officers fired over 650 rounds and the two suspects fired over 1100 rounds. So having some extra mags isn't necessarily a bad idea. Obviously, not at the expense of other critical equipment.
I know that ALERRT (active shooter training) encourages officers to carry at least 100 -120 rounds. I personally carry two spare rifle mags and two spare pistol mags on my everyday-wear carrier and five spare rifle mags on my throw-on LBT carrier. Just my thoughts and something to consider with plate/carrier selection. Excellent info in the post.