The number of PLC-Air v. PLC-Ground varies from year to year depending on the needs of the Marine Corps. You can pretty much count on that when considering your career path while a Marine. If they need pilots, the number of spots go up, and vice versa. PLC is a little different because they know that they will attrite a certain percentage of candidates. They look at how many positions they predict being open and try to balance that with the number of seniors coming back/those accepting commisions/incoming juniors. I'm sure there is some high-speed formula locked up somewhere at HQMC, but that way above any of us. Again, your OSO can probably fill you in as to your chances of getting selected for whatever it is you want.