Well, that depends on the specific type of bus. Smaller buses may have four wheels. Six wheels is pretty common for buses. Some may have eight or ten wheels, depending on the vehicle's gross weight rating.
Buses can have as few as Three wheels in places like IndiaMost buses in North America can have between four and eight wheelsCity buses mostly have six wheelsGreyhound type buses mostly have Eight...
Most school buses have two axles and four wheels or four sets of wheels. i.e. one wheel in each front corner and a pair of wheels on each side in the rear. See this link...