If my opponent has Ampharos with "Electromagnetic Wall" in play, when I attach a Double Colorless Energy to one of my Pokemon, do I place 3 or 6 damage counters?
You place 3 because there was only one attachment, even though the attachment provided multiple energies.
Source: TPCi Rules Team (2012-09-13)

Relevant cards

Note: All applicable cards may not be listed below.
BW6_40