It really all depends on condition of paint, whether your doing a heavy cut or a polish, applying wax or sealant, etc,
For heavy cutting I can get away with 2 or 3 pads if I clean them on the fly. For polishing I've always been able to get away with 2 pads. Fine polishing, jeweling, or applying wax or sealant should only take 1 pad.
The quality of your decontamination work and the hardness of the paint your working on will also affect pad usage.