Enough to kill Orcus. I actually did the calculations as a thought experiment in the early days of 4e, using only material from the PH. I managed to come up with a party of 30th-level characters who could kill Orcus in five rounds provided nobody rolled a 1. Now that I've got Adventurer's Vault, the FRPG, Martial Power, PH2 and Arcane Power, I'm fairly sure I could do it in less than five rounds. While I won't go into the details, the key elements of the strategy were:
1. The warlord's Tactical Presence, which grants a bonus to attack rolls when an action point is used. This stacks nicely with some paragon path abilities which grant a further bonus to attack rolls when an action point is used.
2. Hand of the Gods (Paladin Encounter Attack 17) to give an early boost to attack rolls.
3. Armor Splinter (Ranger Encounter Attack 13) to lower Orcus's AC.
4. Lead the Attack (Warlord Daily Attack 1) to boost everybody's attack rolls against Orcus.
5. Chains of Sorrow (Fighter Encounter Attack 13) to lower Orcus's defences.
6. Plague of Doom (Cleric Encounter Attack 13) to lower Orcus's defences.
If I were to re-do the party now, I would give everyone Weapon or Implement Expertise, and use an eladrin warlord with the Spiral Tactician paragon path.