I have ~4+ units in one city with an airport and I want to airlift them to another city w/ airport. the game will let me do that only a couple times in one turn. Is this a bug, or is there a limit on how many times you can airlift?
Once your civilization has discovered the requisite advance, you can build Airport improvements in your cities. Once you have two or more of these, you can airlift one unit with the Airlift ability per turn out of each.