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Eighteen wheeler is a familiar expression, that most people will recognise as an Americanism referring to a big truck.

The English count the number of axles for legislative purposes, so eighteen wheels adds up to four twin wheel axles and one single wheel steering axle, ie, a five axle articulated vehicle.

The original American eighteen wheeler is more likely to be a 6x4 (6 wheels altogether,4 of which are powered) or 6x2 tractor unit pulling a tandem axle semi-trailer

A popular European rig used to be a 4x2 tractor unit pulling a tri-axle semi-trailer, also eighteen wheels.

Trailers nowerdays tend to have "super singles", wide wheels instead of doubled up wheels. Tractor units often have two steering axles and one twin wheel drive axle.

So, a modern eighteen wheeler only has twelve wheels, if you count like Americans

But then if you use the English 4x2 type designation the American eighteen wheeler only has ten wheels.

Unless you prefer juggernaut.