If you have no limits on plane types, you can get 50 allies vs. 10 axis; all pilots in whatever is the favorite plane on a side with no one in the less-favored planes; all bomber pilots on one side or the other; etc.
That works against giving the event a historical plane mix; it is not balanced; and you can't come up with a scoring system that is even side to side.
Instead, we estimate how many people are probably going to be in it, judging from the past several scenarios. Then we put in the various plane types that were there, in ratios that make it both historical and balanced.
If more bomber pilots want in than there are available spots, we can add bombers proportionally on each side and adjust scoring proportionally.
If there are more fighter pilots who want in than there are available spots, we'd add more P-47's and more 109G-14's and/or 190A-8's equally to each side.