>> The 4077th was nowhere as big as the real 4 or 5 MASH units in Korea. <<
And for the same reason that every school-based sitcom had eight kids in the class, lined up in two-by-four seating arrangement, instead of the seven-by-five seating arrangements in my real 1980s classrooms. The TV show further reduced the size of the MASH from at least six doctors besides the CO to only three.
I was a teacher for 16 years in Los Angeles, and I absolutely hate teacher movies for exactly that reason. The teacher has one class with maybe 10-12 kids in it, and there's nothing realistic about any of their interactions.