camshafts

Why is it that a DOC produce more horse power then a single cam? At least thats is what I am told, but when i asked him why, he didn't know.

They don't always make more power.
They have several more internal moving parts which require power to drive. Honestly, they also aren't as efficient as a typical pushrod motor.

They do make good power though. It all depends on heads, cam(s), etc...

One advantage of dual cams instead of a single, is the use of more valves. Typically, 4 valves per cylinder will have improved flow over 2.

Sorta.

DOHC limits how much lift you can run. Most stock DOHC motors will move more air. Most modified 2-valve pushrod motors will move more air.
Depends on the application.