On the Korean map, I have reached a hard limit of 40 airports. That limit includes the auxiliary add-ons (i.e. STRIP7A.OT).
There is no getting around it. If you try writing in extras into the mission file, it'll be ignored. If you go over 40 in the map file (NSK.MM), you'll get a memory error crash.
I think the best way to handle it is to decide the top 30-some to keep permanent, then store the rest in a text file, such as the scenery text files I use (see link below), and put those in the mission file when needed.
https://myplace.frontier.com/~usnraptor/Fighters%20Anthology/Maps/Korea%20Scenery.txt