To clarify, 12 is not the cops, not the police. NO. 12 is the narcotics officers that show up at your door, or the drug task force in other words the DEA (Drug enforcement Administration). They combat drug smuggling use in the U.S. Aka why in the song fxck 12 by migos he says "throw that shxt throw that shxt 12 outside" refering to throwing the drugs because the DEA or 12 is about to kick the door down.
It's 12 Districts in the greater city of Atlanta. There are always 12 officers on patrol at all times in each district and each officer works 12 hour shifts! That why ATL says "12" when referring to police.