What is the maximum party size and the corresponding monster buffs?

Solution 1:

As of the 1.05 patch on 10/16/2012, monsters gain a 70% HP increase for each additional character. So a 4-person Inferno party would face monsters with 210% more health than a solo character. (Prior to 1.04, the % increase per character was higher on higher difficulty levels).

Patch 1.08 reduces the HP-buff to 50% per player, resulting in a maximum of 150% more health.

This increase is applied AFTER any increase for monster power level. So at Inferno MP1 with one player, monsters have 150% hit points. With four players at MP1, they would have (150 * 3.1) = 465% hit points.

Update: There used to be a bonus to damage on higher difficulty levels, but it was removed in a hotfix on June 8, 2012.

Solution 2:

The maximum party size is four people.^{1 2} There is no party experience split.³

For each additional party member, monster health scales: +75% HP in Normal, +85% HP in Nightmare, +95% HP in Hell, +110% HP in Inferno.^{4 5} (The upcoming 1.0.4 patch changes this. Instead of scaling monster health buffs with difficulty, all difficulties get a flat +75% HP increase per each additional party member.⁶) There are no other party size buffs.

Solution 3:

Concerning the buff to monsters, as of 1.0.8 (currently in PTR):

Monsters currently[1.0.7] gain 70% extra health per additional player in the game. This is being reduced from 70% to 50%. This means in a two-player game, monsters have 150% as much health as a single-player game. In a three-player game, they have 200% as much health. And in a four-player game, they have 250% as much health (compared to 310% on live).

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