1 = Central Division
Adam = Two Man Patrol Car
12 = Basic Car Area of Patrol
Every division in LAPD is further divided by basic areas, in which specific cars are assigned. Usually one of the grid numbers gives the unit area its number.
In the case of Adam-12, there are several stories, of how "12" came to be. One is that Dan Cook, LAPD Liason officer once worked in a unit 12. Another story is that Jack Webb once lived in an apartment 12.