It looks like Joseph Holmes is using medium format cameras, which I don’t have. But using a DSLR, the number of layers per stacked photo depends on a number of factors:
(1) The Type of Lens. If you are using a wider-angle lens, you need fewer layers because each layer takes in more of the subject in one shot. If you are using a macro like from 100mm-200mm you are taking in less of the subject with each photo and probably need more layers. If you are shooting landscapes and not close-ups, then a few layers will do it because you can't see any artifacts, for the most part.
(2) Front to Back. If your subject from front to back is very deep and has layers that protrude over the background (like branches of a tree) and you want the background sharp as well, then you need very short increments between photos or too many artifacts will accrue.
(3) Your Goal. If you want a micro-photography-style shot, where everything is in perfect focus, you may need a focus rail and a great many shots. If however, you are sampling the image to gain an overall impression, then you can experiment with fewer layers and make the resulting artifacts a feature rather than a liability. Focus stacking, by definition, is a sampling technique that is impressionistic, while any standard one-shot photo is a smooth linear transition from front to back, but not with everything in focus.
The bottom line is that you can answer your own questions by actually just experimenting with stacked photos to see what works best for you. If it helps, I have a free ebook on DSLR focus stacking at http:www.macrostop.com.