Many. In fact there are ten of thousands of instrument rated pilots in the world and growing steadily. In the time it took me to write this answer, more than 15 individuals qualified as an instrument rated pilot.
GPS
A small %. If a private pilot was intrument rated they probably have their commercial license so they wouldnt be private pilots they would be commercial pilots
S(he) uses an altimeter which a kind of airborne barometer.
Pilots first instrument used in flying was a compass so they could orient themselves in what direction they've been heading and should head based on time and speed.
They do there are many pilots in the world to do the flights
An instrument proficiency check (IPC) is typically valid for 12 months. Pilots must complete an IPC to demonstrate their ability to perform instrument flight operations competently. If the IPC is not completed within that timeframe, the pilot may need to undertake an additional training or evaluation to regain proficiency. It's important for pilots to stay current with their instrument flying skills for safety and regulatory compliance.
You will need 2 pilots
In 2010 there were 21,063 civil aircraft registered, and about 28,000 Private Pilots, and 10,000 certified glider pilots.
About 20,000 American spitfire pilots died in WW2.
Pilots will reference many instruments throughout their flight. Attitude indicators, speed, altimeters, vertical speed indicators, turn indicators, navigation displays, flight management systems and many engine instruments that display temperature, rpm, various pressures such as oil, manifold and hydraulic. The most important instrument is probably their fuel gauge.
Pilots and sailors first instrument used in flying and sailing was a compass so they could orient themselves in what direction they've been heading and should head based on time and speed.
There is no exact data as to how many airline pilots are in the world. There are an estimated 800,000 in the United States.