Symbolism, Imagery, Allegory
The best thing about the required missions is how they function in the "time" of Catch-22. The number of required missions tells us where we are in time. We know if there are sixty required missions, this is later in time than if there are forty. The fact that we measure time by how worked-over the men are by their superiors says something about the tragic nature of this text, the hopelessness of Yossarian's plight, and the lack of trustworthiness of the powers that be.
But that's not all. The shifting number of missions also reminds us that time is not chronological is this novel. We see the missions rise from fifty to fifty-five. OK, that makes sense. But then we see them getting raised to forty. Which is a kick on the head that Catch-22 isn't chronological.