just did some reading, it's probably reasonable to assume Admiral Schulzski was the only incubus on the ship, however; " manipulate or even control humans, either with telepathy or more direct methods" and "Infestation covers everything from initial implantation to the manifestation of 'telekinetic' abilities."
I'd say yeah there may have only been a few incubi on the ship, but the rest could have been infested OR under the influence of telepathy.
I guess the Donau was a very unfortunate case where the Order was forced to destroy the ship, as all other solutions were out of their timeframe / too improbable...
