Gunship/Gunboats: 1-25, depending on things like cockpit style, origin of the ship, size of the ship compared to others in it's line, and really just what you feel sounds "good" in RP. For example, I roleplay my Liberty Navy Gunboat as having a crew of between 10-15, with room for at least ten Marines who inRP have bunks set up down in the cargo hold. (Some Gunboats are more like "Frigates" than "Gunboats")
Destroyers/Cruisers/Battlecruisers: 150-350, though again this depends on ship size in comparison to others in it's line, and things like the inRP technological capabilities/ranking of the House/organization that's fielding the ship in the first place.
Dreadnoughts: 400-600, though I'd wager there's a lot of wiggle room here, taking into account the different factions that field Dreadnoughts, and their technological capabilities.
Carriers: Across the board I would say that Carriers have a minimum crew somewhere in the high hundreds though if I had to place a number on them I would say at least 1000 crewmen are aboard a Carrier.
Ultimately though, the decision is largely left to you, the people who fly the ship. What I try to do is sit and think about it like this: How many crewmen do I think this ship would need to manage different systems and sections aboard the ship, I'll give an example below, naturally.
Again though, largely this is a decision left up to individuals. Mainly one should just take into account the technological capabilities of the group they're flying for, and how large their ship is in comparison to other ship's fielded by the same group.
Anyway, on to my example!
Libertonian Gunboat with "Full" crew complement:
CO: (Commanding Officer) x1
XO: (Executive Officer, 2IC, Helmsman) x1
Additional Bridge/CIC Staff: Weapons Operator x1, Comms Operator x1, Navigator x1, Systems Tech x1
Medical Staff: Naval Doctor x1, Naval Corpsman x1. (Facilities located in lower decks, near "Cargobay".)
Naval Engineers 2-5x, (depending on if you want just engine techs or guys to repair all systems 8| )
Marine Security Detachment/Boarding Party 5-10x (Bunks located in lower decks, probably bolted into the cargo bay).
This method of course, can be applied to other, larger ships, with different purposes or belonging to various factions as well, I only used a Libertonian Gunboat because it's a fairly easy example, and my Lib Gunboat is something I've done a lot of roleplaying with, so I like to think I have some idea how it works...
But ultimately like I said, it's up to the individual playing the ship, and how you want to play that ship, and what it's inRP purpose is. The only thing that I would consistently take into account is: "What are the inRP capabilities tech wise of the group that I am flying for?"
The higher tech the equipment is, the smaller the crew you'll need, theoretically. (Assuming high tech = automated systems, or systems assisted by AI or VI programs etc etc)
I hope that my opinion gives people some ideas, but even if not that's fine, ultimately it's just an opinion, after all.