Jerry Seinfeld's apartment number on the show is 5A.
The apartment is one of the series' main recurring settings, where Jerry hosts George, Kramer, Elaine, and a steady stream of neighbors, dates, and unusual visitors. It is presented as an Upper West Side apartment in Manhattan, though the exterior building used for establishing shots is located at 129 West 81st Street.
The number was not completely consistent at the beginning of the series. Jerry's apartment number changed during the early episodes, but 5A first appeared in the season-two episode "The Apartment" and remained the apartment's identifying number through the rest of the show. This is why some viewers remember seeing other numbers in early installments.
A related detail often causes confusion: Jerry's apartment is commonly associated with the fictional address 129 West 81st Street, while the actual exterior used for filming is a real Manhattan building. The apartment set itself was filmed at CBS Studio Center in Los Angeles, not inside the New York building.