Joined: Feb 2013
Posts: 1,472
Likes: 11
From: Near St. Louis, Missouri
Bikes: Giant Defy Advanced, Breezer Doppler Team, Schwinn Twinn Tandem, Windsor Tourist, 1954 JC Higgens
An 8-speed chain will also work perfectly for 6- and 7-speed bikes. I'm assuming that by 12-speed, your bike has six gears on the rear.
A 5-speed chain will be too wide to work properly. A 9-, 10, or 11-speed chain would be too narrow and could fall between the gears as Andrew mentions.