Should the Gauge of wire be consistent for DC
Going with the maximum given, 1 Amp, 12V, you can have up to 20 per rail. 17 or 18 as to not drive the rail to the limit. And that's if the combined 12V output is actually 480W, as there might be a limit. If the combined output is limited to say 360W, that means you are allowed 240W on Rail A, or 240W on Rail B, but not at the same time. And 12W is fine on 20AWG