My best is 7 fighters in 1 pass in a P51D.
Sounds incredible? It was a once in a lifetime event:
I'm flying towards an enemy field high when I notice that the darbar starts to grow..
Great I think, some defenders are upping for a fight, they must have noticed me come or something.
I dive down past the cloud layer expecting to see 2-3 planes circling around the ack. Couldn't have been more wrong!
There's a conga-line of a whole mission that just spawned under me, everyone's heavy and so totally worked up with their mission that nobody noticed me screaming down behind them.
I dive down behind an another P51 and start shooting. Incredibly he makes no effort to evade and goes down despite I'm compressing and have difficulty to hit.
Ok, surely he will warn all the 20 friends and I'll be in deep doo-doo right? Wrong. I move to the next plane, shoot him down. Then the next.. Heck I'm flying past fighters on my side that I can't hit due to excess speed and nobody airs a warning! When I'm at the 5th plane someone finally gets a word of warning out and people start dumping their ords and evading - but it's all too late for yet two more.
Then I got over confident and greedy and stayed to turnfight 1 vs 13 and never made it home. But it was a pass to remember!

Now ever since that I've been dreaming of the ultimate: To manage to drop a bomb exactly on the spawn point of a large mission just when they spawn lol!