Other mistake: What happens to Tegan's bag when the plague victim transforms into the cavalier? It doesn't appear to be left behind, even when the cavalier later vanishes.
Resurrection of the Daleks - S21-E4
Other mistake: When the Doctor's mind is scanned, we see his previous incarnations and companions, but Leela is not shown. Katarina is represented by a black and white photograph.
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