Any book called The Hero with a Thousand Faces is likely going to conjure up some kind of Voltron-on-steroids image of a giant monster dude with a thousand little heads making up one big one.
Luckily, Campbell's big into symbols, so we don't need to take such freakiness literally.
The title's a nice shorthand to explain what Campbell's talking about. The countless stories he cites in the book—the avalanche of examples that accompanies each and every point he makes—are all variations on a theme. Every hero in every story ever told is different, and yet he or she still represents the same basic story underneath. It's all the same skeleton. The surface details are different just to match the time and the culture, and to help people better understand the underlying message.
So, there is really only one hero. S/he simply wears a thousand different faces…with new faces cropping up every time someone tells a new story.