I suppose for a simulation, one or two kills per mission would be pretty good, as it demonstrates that you can hit what you go after enough times to down it without running out of ammunition. Although having said that, the enemy AI in the IL2 series is not the most challenging there's ever been in a sim.
Unless of course you could shoot like this guy, then two per mission would be about average:
http://en.wikipedia.org/wiki/Hans-Joachim_Marseille
Mind you, like a lot of aces, he couldn't land worth a damn.