Nobody can answer a question like that without knowing things like the application (home stereo/theater, auto, PA, what?), the output capabilities of the amp(s), speaker impedance, etc. In other words, more info needed.
I know there are MANY, often contradictory, opinions about this but . . . to my thinking, 12AWG is probably overkill in that application. 14AWG or 16AWG should be adequate and, like "DF96" said, you can salvage it from the old extension cord you ran over with the lawnmower.