It is a combonation of urination and defacation in the same restroom visit. Derives from from the words number 1 and number 2.
James: Hey Travis you were in there awhile. Did you do a number 2?
Travis: No man, a number 12.
James: Why do you give it a special name? Everyone pees when they poop most times...
slang for head, dome, oral sex, or pp in the mouth. Originates from mcdonald's and their #12 Dome piece chicken
Yo nigga i got number 12 all night long