all depends on the individual ship. Most people like to say they have massive crews on their capships, I always took the point of view that as technology advances, the need for excessive crewmembers decreases. You dont need to have people pouring coal into your boilers anymore, nor does it take 3 men to man a turret, they're computer controlled.
For most gunboats, I typically estimate a crew between 5 and 20 people, with larger gunboats leaning towards the latter. You also have to take into account the ship's mission or task at hand - ships which are designed for long, deep-space patrols will take multiple 'shifts', so you have to double or even triple the crewmen onboard. For a small gunboat running 3 shifts, I'd say 15 people average.
Cruisers I typically estimate at twice the amount necessary for gunboats, but again it depends on the size of the cruiser and it's primary mission. Most cruisers are probably going to be in space and operating for longer periods of time than the short-ranged gunboats, so they all need at least a double shift. Smaller cruisers like the kusari destroyer, resheph, thresher, etc. probably only run with 30-40 crew members average, while larger variants like the rheinland cruiser probably have 50-80.
Battlecruisers are larger than cruisers, but crew sizes probably dont vary much. I'd put all of the 'standard' battlecruisers (liberty, img, and bhg) at roughly 80-90 crew members max. The order light carrier obviously has much more than that, due to the need for a full array of fighter pilots and extra crew members tending to the launch bays and ship outfitting/repair.
Typically I claim battleships have around 150-200 crew members, based on size. Some larger battleships, like the liberty carrier, rheinland battleship, kusari battleship, or dreadnaughts more likely have around 300 crew members. Finally, colony ships like the zoner juggernaut are more likely operating with anywhere from 500-1000 crew members, most of them would be colonists/civilians, however, and not necessary for ship operation.