In modern designs, the # of +12V rails should not matter at all. In quality units (like Antec and Corsair, but not the original Coolermaster) the OCP will protect any rail from a short, while allowing it to deliver enough power to whatever is attached to it. In a single rail design, there is no OCP, so a short in one device may just look like a heavy load to the PSU, until the faulty device (or its power cable) smokes or burns.
Edit: Considering that Coolermaster still sells a number of liar-labeled PSUs, or ones that are missing protection circuits they claim to have (hardwaresecrets.com has the reviews), I will not buy ANY of their products. Check out Rosewill's gaming cases, they're all pretty nice; or the Enermax Hoplite.