New York Police Department and the FBI have done a good job of keeping statistical records concerning police shootings. In fact, compared to NYPD, the FBI is a little light on data. New York has been tracking shootings for more than a century.  The point is we know that for good or for ill, four rounds pretty much wraps it up.
Bear in mind that most shootings occur within 6′ or less, so it makes sense  that  the ammunition expenditure would not be all that great.

Today, most peace officers carry a loaded pistol and two magazines.  That’s about 50 rounds. In my opinion its too much.  The likelihood that a reload will be required is more of a serious possibility than a probability but a reload will almost certainly be due to a mechanical failure not the exhaustion of the ammunition supply. The great advantage of the pistol is that it allows a quick reload with less training effort than required for a revolver but a good revolver shooter will still beat a mediocre pistol shooter in a reload.  For a true pistol advantage, the defender needs more magazines with less ammunition. Everyone seems to agree that 2 extra magazines or speed-loaders are sufficient but why does a defender need to carry a box of ammunition?
Heavy magazines are more susceptible to breakage than those which are lightly loaded.  Single stacks take up less space and minimize the snags that happen when those wearing a gun belt are forced to squeeze through tight openings.  If you wear a loaded gun belt often enough your back will not thank you for it–lighter is better and quicker.  Smaller grips and smaller magazines are good for both speed and retention. Safety is enhanced when a pistol can be operated from the shooting grip rather than having it twisted about in the hand.  Size matters. Revolvers were discarded because of the fear of high-capacity weapons but even the bad guys ammunition average in shootings hasn’t risen. Its not a question of the presence of ammunition or the mechanical capability of the weapon–it’s a matter of time. That hasn’t changed.

Let’s not go equipment crazy to our own detriment.