If the semi-auto ar-15 pattern rifle is a "military style" rifle, then our military or other world militaries must have many in use, right?

I wonder how many semi auto M4's or M16's there are (I already know the answer to that.... M4's and M16's are a spec, and thus there is no such thing as a semi-auto M4 or M16).

As to the "designed to kill as many people as possible" argument, if that's true (and it sort of is*), then why do the police carry them? Are they planning on "killing as many people as possible"? Or are they just trying to ensure the best possible odds at stopping a threat*?

*or "threats", if there are 5 bad guys, you want at least a few shots for each, right?