I would say somewhere around 2 flyers or 4 monsterous flying creatures. I would say 2 flyers because anything morethen that is hard to destroy unless you allocate a bunch of points to a unit wth AT and skyfire.
While i say 4 monsterous flying creatures because you dont need AT weapons to kill them, you can do that with lasguns by just causing grounding tests which means potentially any unit can kill them.