You're in luck. My friend has a copy.
Anyway, a Sword class Frigate carries around 26,000 crew.
The Sword class is also a fair sized bigger than a Cobra.
A Lunar Class light cruiser is crewed by 95,000 men.
Ah, GW minimalism at its best. As I cannot comfortably state the number of ships of each class in sub-sector fleet that would put the figure for imperial naval crew much lower than I estimated. I would presume however that the larger Apocalypse and Imperitor Battleships as well as the vasty oversized megaships that pop up in fluff on occasion would have larger crew needs. For the purposes of simplicity lets assume that the crew needs of an Apocalypse class are only slightly greater than that of the Lunar Class, lets say 110,000 men.
50-75 ships of various classes crewed by somewhere between 25,000 and 110,000 would put the sub-sector naval staff at between 1250000 and 8250000 people per subsector. So between 312500000000 and 8250000000000 naval personnel in the empire, as a rough estimate of course.
None of this accounts for the staff necessary for drydocks or star-fortresses in orbit of planets.