How many level 9 PCs needed to be able to kill an adult red dragon?

[KorvinStarmast]

Any 1 v 6 fight is absolutely in favor of the 6 - it really is that simple. However, IF the dragon is clever (hi INT and WIS stats would lead you to believe it is), it ought to take max advantage of its LA and superior fly speed w/ breath weapon and frightful presence to help mitigate the party's numerical advantage.

Personally I always look for good opportunities for my dragons to grapple one PC and fly off with them - it's a nasty option.

I did this last year. The rest of the Players were a bit non-plussed at first. Note that the soon-to-be-grappled player had told me that he was going to be out for a few weeks, and I stumbled over this idea for how a dragon becomes a threat to the party, but also flies away when the action economy you mention leaps out at him and he's like "this isn't working!"

Dumb luck on the timing.

The other players hunted the dragon down and (through good planning) killed it. Adult green dragon. Yes, the monk eventually got through the LAs and stunned it ... so it goes.