I believe it is either 5 or 10 building hit points that are removed by Sabotage spies. Typically 5. It may be that the damage is a random choice betwen 5 or 10, or it may be building specific.
Regarding the airfield it will take several sabotages to render it unusable. 2 will still leave it usable at 50% damage and capable of flight operations with a 30 minutes refueling time.
The fort will take 4 sabotage operations per level of fort.
