Xfire0125- Short answer: "A lot".
I used at least 8 pads via Flex when doing my last major correction, and those were 6.5" ones!
With 4" pads (and yeah, I think that's the way to go via PC unless you want to try SurBufs) I think you'll need many more than you'd ever expect if only because they need to be not only clean, but *dry*.
Depending on how big the vehicle is, and how much correction/work you'll be doing, you might want to plan on at least one 4" pad per panel, with a few extras just because [stuff] happens.
I'd rather spend money on extra pads (not like they go bad or anything...) and be prepared so the job goes as smoothly as possible. Saving money might end up trading dollars for frustration and I'd rather do without the frustration.
rtom- Working via PC, with M105, I've found that any pads larger than 4" are a waste of time on most of my vehicles. (No, I haven't tried the SurBufs.) I know that's different from what others sometimes experience, but hey...it either works or it doesn't :nixweiss