How many pictures should be included along with writing?

It depends on how complicated or delicate the procedure is—and how versed your target audience is with the procedure.

For example, at iFixit we write repair manuals for consumer electronics. Our guides and manuals are designed so novices can easily use and understand them. Naturally, we don't want our readers to damage any of the delicate components in their iPhones or their MacBook Pros. So, we supplement our textual instructions heavily with pictures. Essentially, every time the repairer's hand moves, there is a picture to document it. We need the reader to see just how a component slides off the connecter, or how a tiny tab swings open. Without that level of detail, novice repairers might just pull a component off the motherboard and completely ruin their device.

Of course, not everything requires that much visual explanation. A general rule of thumb: Use pictures when words won't suffice on their own, and your text needs that extra level of visual clarity.