The Centipede 416 uses the first 3 board numbers: 0,1,2 for the battery changer as board 0, and the built in eStim channels as boards 2 and 3. So you will want to number pods starting at 3 up to a maximum of 15. There is no requirement for Pods to be sequentially numbered, and you can certainly setup a system with missing numbers. Pods do not have to be connected in numeric order either.
The board number is determined by a small four position slide switch inside the pod, so the setting can be changed after shipping. As long as each pod has a unique number, it will work fine.