How many rounds do you plan to use?
In Iraq having 12 magazines was good as you could be fighting for hours and have little to no back up with guys using machine guns.
Here a few magazines should serve one well if the plan to aim and account for their rounds.
I dont see officer taking AR/M4's to most calls, so they dont need the extra magazines and ammo weight/space taken up. A few guys at work carry a single 20 round mag.
Why not have something like the Ready Mag http://www.redi-mag.com/index.php?l=product_list&c=9
that would give you two magazines every time you grab the rifle. That would give you 60 rounds, more than most carry for pistol ammo. Then if needed get an extra magazine mounted to the outside butstock so you can have 90 rounds, that will be with the rifle. If need be on a active shooter call you could toss some extra rifle mags in the side cargo pockets giving you 150 to 210 rounds. 210 rounds in an Infantry soldiers combat load to fight in battle with. Thats facing a few enemy that are well armed and shooting back.
Me, I have two rifle mags with the rifle, extra on a raid vest. If stuff goes bad I am taking more equipment from the wounded and dead.
Are the streets getting that bad or are officers looking to get in trouble using up what they have? Its like when the semi-auto pistol came to be issued, officer shootings went from 3-4 well placed shots to 9-12 shots, and now its reloads, soon it will be suppresive fire with Machine Gun teams covering.