Whether one pad will be enough to heat the entire encosure depends entirely on what size and type of enclosure you use, and what the ambient temps in the room are.
You may have to do some experimenting to see what works best in your situation. Your BEST bet is to start setting up the environment well before you plan to bring the snake home. Once you are certain everything is working properly, then you get the snake and can feel confident he's going into a perfect home.