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

[Aimeryan]

Again though, you're really just treating it as stand and fight type of combat on both sides. Dragons fly, fast, and have a ranged huge aoe weapon, and this one has 16 Int - it isn't going to stay there trading blows when it can out speed everyone but the Hasted Rogue (which can't retreat far enough away to not suffer the multiattack + a Tail Attack). Also really surprised that with a 60ft diameter circle it can not hit more than two people, but the whole party are all able to attack a high up dragon the same round - it seems like it firing the breath horizontally and then heading into the middle of the group (for some reason?).

Some of the damage also seems off - can't place the +9 on the Rogue's attack, also Fighter seems to have an added damage point per attack but that probably just a +1 Longbow. Fighter should hit about 1 in 4 on average with Disadvantage, maybe 1 in 3, so either getting lucky or again the dragon is staying close for no real reason (it can either just outrange and breath or take on the Rogue essentially solo). If the Dragon recognises a Hasted opponent (which should be obvious to a 16 Int dragon) it is likely to either deal with that opponent first or take out what appears to be the caster (hint, not the Fighter in plate and a Longbow, not the Barb with axes).

Honestly, this dragon is fighting like it has an Int of 6 or is a World of Warcraft boss. It has complete control to outmaneuver the enemy and the intelligence to take out the casters to drop the buffs, after which it is basically uncatchable flying around breathing to death the rest of the party at leisure. Or, fly away and deal with the Hasted Rogue solo (which without Fly doesn't have the speed to retreat safely after a melee attack - and one round of the dragon getting to do it whole attack routine soundly kills the Rogue, two rounds maybe if it needs to use Wing Attack to close), then come back at leisure to mop up the others.

My last analysis, I did forget about Uncanny Dodge - that would double (less any lucky breaths) the number of rounds required to kill the Rogue (because the dragon can only Ready one attack), which should grab a win for the party (likely the dragon flying away after just managing to kill the Rogue rather than risking low HP on potential Longbow crits and low damage but long range spells when it comes in for a breath attack). That said, I also presumed the dragon would always move for the Booming Blade, which is not actually needed as the rest of the party cannot get in range after just a short flight upwards, and it would not be able to outfly the Hasted Fly Mobile Rogue - so why move once reached a decent height, since it can only attack anyway by Readying for when the Rogue gets in close? That would increase the number of rounds the Rogue needs to kill the dragon (or get it low enough to force it to flee after killing the Rogue), so putting it back in the not likely to succeed category (lucky Rogue crits over and over aside, naturally).