Although the Lieutenant is actually sent to the police station in precinct 9, division 13, the sign over the door states it to be division 14.
The assault takes place on Precinct 9, Division 13. There is no "Precinct 13" in the film. At first John Carpenter wanted to call the film "The Anderson Alamo" (the original title of his screenplay), and at one point he changed the working title to "The Siege." CKK, the film's distributor, was responsible for the misnomer; it rejected Carpenter's titles and came up with the name "Assault on Precinct 13" (which it felt was more ominous sounding) during post-production.