Which of the four named U.S. sitcoms had the most episodes over all of their respective seasons?

Of the 4 sitcoms named, "Cheers", set in a Boston bar, had the most episodes with 275 half-hour episodes, across 11 seasons, from September 30, 1982, to May 20, 1993.

In terms of episode numbers, "Cheers" is followed by “M*A*S*H” (an acronym for Mobile Army Surgical Hospital), which ran for 11 seasons from September 1972 until February 1983. "M*A*S*H", set in a U.S. Army field hospital in the Korean War, had 256 episodes.

Some 20 years after "M*A*S*H", “Friends” began with its pilot episode, which was broadcast on September 22, 1994. The series finished its ten-season run on May 6, 2004, with 236 episodes broadcast.

Fourth on the list is “Seinfeld”, which ran for 9 seasons from July 1989 until May 1998 and had 180 episodes.

The sitcom, "The Adventures of Ozzie and Harriet", aired on ABC from October 3, 1952, to April 23, 1966, and lasted for 14 seasons and produced 435 episodes.

The 15th season of "It's Always Sunny in Philadelphia" in 2020 will make the series the longest-running live-action sitcom in American television history but with far fewer episodes than "The Adventures of Ozzie and Harriet". It will be closer to "Seinfeld" in episode numbers.