If there are two Gengar in play with the "Gnawing Curse" Ability, how many damage counters would be applied if the opponent attached an energy card to one of their Pokemon?
They would place 4 damage counters on that Pokemon.
Source: Crimson Invasion FAQ (2017-10-20), TPCi Rules Team (2017-11-02)

Relevant cards

Note: All applicable cards may not be listed below.
SM4_38