Other mistake: When the Doctor pulls Zoe and Jamie back into the TARDIS, look at the scanner screen. It has the words 'Producer Peter Bryant' written on it.
Other mistake: The Master says he wrote 5,000 words a week for 25 years and Zoe says "but that's over half a million words." Yes it is, to be exact, six and a half million words. (5,000 x 52 x 25 = 6,500,000).
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