Usually 3 or 4, depending on who has what skills, etc... At least until you get above 5,000 tons.
This is because a large part of things is automated, so you only need to man the critical stations during periods where your computer program won't handle it, such as combat, docking, disembarking, Jumps, etc...
So typically the essentials are a Astrogator/Navigator, Pilot, and Engineer at these times, and gunners, if you have turrets, during combat.
Additional skilled crewmen would be nice as well, but those are the bare essentials. Ideally you want to have someone skilled in all areas you may need repair, so you would like to have a Maneuver Drive Engineer, Jump Drive Engineer, Electrical Engineer, Comms, Computer technicians, etc...
Plus if you transport passengers you want someone with the skills to take care of them. So Ideally you will have all of these skills wrapped up in as few people as possible.
Then, with small ships like the Scout, you have just one person, who is only definitely not going to have a wide enough range of skills, so that is why you have the "AAA Space Travel and Insurance Agency" to come and give you a tow when your Scout breaks down and you can't fix it. Or bigger ship, when it is beyond your crews ability to effect repairs.