Okay, so thinking it through, 4 could theoritically be enough. 2 ground witches and 2 air, one night, one ground attack. And I just found my ground attack plane/weapon combo. Its a mean one. Big, heavy and very powerful. The BK 7,5, 75mm air-to-ground auto cannon. Might be too big, though it was carried on a conventional air craft, and could be mounted on a Stukka.