I'm trying to build a computer, I am considering SLI crossfire (two video cards), so do I need to have two 12V rails? One for each card? Also, does any other component of a computer use 12V rails other than the video card?
The answer to your question is no. A high quality power supply with a single large +12 volt rail is okay.
It's usually the very high end, high wattage power supplies designed for tri and quad video cards that have genuine mutiple +12 volt rails. Power supplies for the typical user and typical gamer generally do not have mutiple rails. Those being advertised as having mutiple rails might best be described as having "virtual rails". It was an advertising gimmick. There was a big stink about it a few years ago.