Just as it says. Achievements for airlines who employ the most staff. Should include a stipulation that you can't be more than a certain percentage over-staffed so a player couldn't hire to 500% just to get the achievement and then fire everybody (or Bankrupt) after earning it.
And just to balance things out and add to the "worst" achievements, maybe achievements for the most over- and under-staffed airlines too. And most over- and under-paid employees.