AfterMASH ran for two seasons. The M*A*S*H spin-off premiered on CBS in September 1983, shortly after the original series ended, and followed Colonel Sherman T. Potter, Sergeant Maxwell Klinger, and Father John Mulcahy at a veterans hospital in Missouri.
The first season contained 22 episodes and initially performed fairly well, finishing among the top 15 network shows of the 1983–84 season. CBS renewed the series, but moved it to Tuesday nights opposite The A-Team for season two.
That scheduling change, along with declining ratings and several creative adjustments, hurt the show. The second season was cut short after only nine episodes, and CBS canceled AfterMASH in 1985. In total, 31 episodes were produced and broadcast.
A common mix-up is counting the short second season as a separate failed experiment rather than as part of the series run. It still counts officially, so the canonical answer is two seasons—not one, three, or four.