Plot hole: Yartek tried to disguise himself as Arbitan by wearing Arbitan's robes and pulling the hood over his giant rubber head. Strangely enough, no-one seems to notice that "Arbitan" is now suddenly speaking with a completely different voice, and now has a two-foot-tall-head.
Plot hole: In episode 4, we're informed that Susan and Sabetha traded their travel dials for firewood, and Ian trades his for furs. They do not take them with them into the caves later on. This only appears to make sense from an out-of-universe perspective: Terry Nation needed the characters to part with their travel dials, so they couldn't have just escaped after collecting the key.
Answer: In 'The Five Doctors', three separate Cyberleaders are definitely used. So it's likely that Cyberleaders are like unit commanders, of which a fair-sized army might have several.
Daria Sigma