It is called a semi because the trailer is a semi trailer. a full trailer would have wheels at both ends and a tongue like a child’s wagon. An 18 wheeler is the most common form with 8 wheels on the rear of the tractor and 8 wheels on the back of the trailer but there are other configurations. When I drove thruway tandems on the NY Thruway and Mass Pike. I was usually driving a 34 wheeler, however sometimes a tandem dolly (8 wheels was not available so had to use a single dolly (4 wheels).
I preferred the superior tracking of the 8 wheel dolly.
When I pulled 3 short trailers in Ohio, I was driving a 30 wheeler.