Each country has different laws on this subject.
I have worked in 100,000 watt FM stations which is the limit in the US.
The height of the antenna is very important in broadcast FM. At 100mhz the signal is mostly line of sight. My antennas were 200 to 1000 feet tall. My little 20,000 watt station reaches as far as I can see form the top of the antenna with a good signal and about twice as far with a week signal.
The little 100 watt station reached as far as I can see with a OK signal. That area is very flat and with a 500 foot antenna we reached 25 miles.