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

[Hael]

As usual with these sorts of tasks, the devil is in the detail. How much scouting/divination is available? What is the geometry of the lair and lair actions available to it. Can the party generate surprise or at least get close enough to the dragon to catch it on the first turn. Are there other threats. What sorts of magic items does the party have available to it. Etc etc

Off the top of my head the fight itself doesn’t seem that tough for a well geared prebuilt and optimized party (4 lvl 9s is more then enough in the right circumstances), the hard part is preventing the dragon from simply running away once it is aware.

Usually that means you need some sort of dedicated grappler that can run down the dragon and down it on the first turn. So potentially lots of movement required (Eg phantom steeds/mounts etc), it will require the ability to fly and grapple a huge (strong) creature as well as winning first turn initiative. Thats annoying, but not that hard if we were allowed to build a character in the whiteroom and scout/prepare beforehand as grapple checks can become quite ridiculous (and we can procur ways to give many rerolls to that check)

As for the statblock itself, certain classes are going to do very well here. Twilight clerics will severly restrict the frightened gaze condition for instance. Lots of ways to procur resistance to fire and/or immunities (absorb elements for instance). Lots of ways to block the cone attack (various wall spells etc). Lots of ways to nova damage pretty hard to take it down pretty quickly. You could also probably restrain it with overlapping walls of force depending on the geometry of the room (if you could force it into a corner for instance).