I wonder, if a broadcasting radiostation can detect, how many receivers are tuned on it, and if it possible - how exactly this detection is happening?
No, that's not possible. There is no practical way to determine whether the transmitted power is usefully received by an antenna, or just absorbed by the ground.
It is possible to detect that a superheterodyne receiver with a known intermediate frequency is powered on and tuned to a particular frequency, because the receiver's local oscillator radiates a small signal. In the UK there is a tax on televisions, and the local authority in the past used television detector vans to detect unlicensed televisions in use, by comparing addresses with televisions running to the database of homes that paid the tax. But scanning for tiny local oscillator signals can only be done in a small local area, because the background of RF noise would swamp any attempts at large-scale detection. Besides, radio design has evolved and receivers use many different architectures now, so superheterodyne receivers with the same IF are no longer universal.