Depends on how many sensors and moving parts. A single microcontroller can handle several servos and multiple sensor inputs. Depends on what kind of inputs you have and what microcontroller you use. For really big programs like roborealm for camera analysis or complex navigation using GPS and location sensors and a large amount of sensors you might need more then one microcontroller. If you do you still do not need a PC because you can dedicate a separate controller to coordinate them all. But those are large projects, your standard small robot project only needs one controller.
