Two HARMs can be mounted on either F-16 Fighting Falcon variant, on pylons 3 and 7. They are highly useful for many missions when equipped on this particular aircraft, especially the first five, in particular Mission 5 itself.
Up to four HARMs can be mounted on the F-35 and X-35, on pylons 2, 3, 7, and 8. However, it is only recommended to use these when replaying earlier levels, such as Mission 5, to take advantage of the F-35's STOVL abilities in order to outrange anti-air weaponry. It is not actually recommended to equip an F-35 with AGM-88's.