It really depends. There are two types of AA guns, big guns for shooting at high-altitude bombers, and smaller guns that fire quickly for low-altitude attacks. It doesn't exactly take a lot of low altitude guns to stave off an attack by a smaller group of ground attack aircraft. The bigger guns had a low rate of fire and were primarily restricted to fighting off strategic bombers. Of course, I think the effects of airfleets should be more abstracted, though, so what do I know?