The short answer
In the EU a category B licence covers a vehicle up to 3,500 kg and 8 passengers; towing a caravan above 750 kg unbraked normally needs category B+E. In the US there is no single federal rule — every state sets its own towing rules, and most states require a special endorsement above 5 tonnes combined weight. In Australia a car licence (class C) covers towing up to 3.5 tonnes, and heavier combinations need a higher class.
Treat this as orientation, not advice: confirm the exact class with your national licence authority before you hitch up.
Europe: categories B, B1, B+E and the auto-combination
Category B covers a motor vehicle up to 3,500 kg GVWR with up to 8 passengers. It also covers towing a trailer up to 750 kg unbraked. As soon as the trailer is heavier than 750 kg, you generally need category B+E, which requires a separate practical test. An alternative introduced for category B holders is the 'auto-combination': a car plus a braked trailer up to 3.5 tonnes on both sides, giving a maximum combined weight of 4.25 tonnes, subject to conditions and a short theoretical course.
Campsite and ferry operators do not care about the theory. What they care about is whether you can legally drive onto their access road, so keep the licence category and the vehicle documents together.
United States: state by state, not federal
There is no single American rule. Class C licence holders in many states may tow a non-commercial trailer up to a state-specific limit; above that, and above roughly 5 to 6 tonnes of gross combination weight, most states require a commercial endorsement. A driver under 21 holding a commercial licence is generally prohibited from towing a housing trailer. Commercial classes A and B require a CDL, a medical certificate and, in most states, separate air-brake and combination-vehicle tests.
Practical consequence: your licence is only one of the three limits. The others are the hitch's rating, the tow vehicle's rating and the trailer's brakes.
Australia, China and Japan
In Australia a class C licence allows towing a trailer up to 3.5 tonnes; a heavier trailer needs a higher class, and states differ on the exact rule and on age conditions. In China the C6 class, introduced in the 2020 driving-licence reform, allows towing a trailer with a total mass of 4.5 tonnes or less, provided the holder has held C1 or B2 for a year or more. In Japan, a normal licence permits towing a light trailer after a procedure with the traffic authority; larger combinations need a separate towing licence.
If you are importing a rig or planning a multi-year trip, ask the licence authority in writing and keep the answer.
The three numbers that actually limit you
Towing capacity is not the deciding number. In order of importance: (1) the tow vehicle's kerb weight and its maximum tongue weight, usually 10 to 15 percent of the trailer; (2) the vehicle's maximum braked trailer rating, which falls dramatically on slopes and in wind; (3) the hitch's rating and the trailer's own axle capacity. A 2.5-tonne trailer behind a 1.8-tonne car is the classic mistake, and it fails on kerb weight alone.
Weigh a loaded combination at a public scale. It takes ten minutes and settles every argument.