Why 18 wheels on a semi truck?

i would suppose its because the weight in which they carry must be distributed on 18 wheels instead of 4, 4 wheels would not work

Each wheel has a specific load rating as does the axle. The more tires you have hitting the road at a given time helps to reduce the stress on the truck/trailer and the road. The more wheels a vehical has, the higher (in most cases) the gvwr of the vehical is.