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.
1 - ADAM - 12 is the number of the car the fucking police drove in an old ass 60's tv show. When we trap and the fucking police come round we holler 12, cause motherfuckers did on the show. It means that the fucking police are coming around.
10-12 is a cb-radio 10 code which means "visitors present". It was a way to let someone on the other end know to be careful on what they said. Today it can be used to warn someone to stop talking about something or to be careful what they talk about at that moment.