The frequency used by pilots depends on the aircraft the pilot is flying. Different types of flying require different frequencies. For example, if you are flying a private fixed wing aircraft then the frequency you have to use is 122.75 MHz. If you are flying a helicopter then this changes to 123.025 MHz. A pilot in a civil aircraft will likely use several frequencies in the 100MHz band. This has 760 allocated channels between 18.0 and 136.975 MHz.
You might use different frequencies depending on the communication you are trying to convey, such as if it is a distress call if you are communicating with a flight watch, and if you are trying to reach air traffic control. With this in mind, pilots of these bigger planes for carrying lots of civil passengers will likely use a large number of different frequencies.