Question:
Can any number of air operations (with an unlimited number of air units) originate from the same airbase and/or all-land clear terrain hex in the North Africa or Britain holding boxes?

Answer:
Yes. airbase capacity does not affect the number of missions that may be initiated from an airbase. Since the entire off-map box is an unlimited capacity airbase, all air units therein become operative during initial phases; then they may fly missions from any hex that qualifies as being part of the off-map holding box, evenif, say, all units take off and return to the same single hex.

Source:
TEM 71