How many stand by engines does a airplane have?

In the Air, the Plane flys on the engines on the wing. If an engine goes out the engine stays out and it flys on the remaining ones. On the ground the Airport may have a few engines available to swap out if something is drastically wrong with the engine. Commericial Passenger aircraft have an Auxillary Power Unit (APU) that is a turbine engine that provides electrical and hydraulic power for emergencies or while a/c is parked on the ground. The Boeing 747 has its APU mounted in the tail--that is the exhaust you see when you look at the hole in the tail. Boeing 727 have an APU mounted in its wheel well; its inlet is on one side the the exhaust is on the other side. Usually on the ground, the aircraft is plugged into ground power carts and air conditioning carts, but the APU can be used.