Answer:
The number can vary with design. For a two axle bus, there would be two tires up front and a pair of dual tires on rear, for a total of six tires.
For a three axle bus, it was common to have the third axle be non-powered "idler" wheels that provided weight distribution. The third axle would have two wheels, for a total of eight tires on that design.
I believe that there are bus designs that have two powered axles rear, which would provide a total of 10 wheels on the ground. There were even foreign buses that had two axles for steering and weight distribution, which made for an odd looking vehicle.
Unfortunately, there is no simple answer; the answer depends on the bus design.