How many people were listening to the radio when World War 1 was declared?

None. Commercial broadcast radio did not begin until about five years after WWI ended. There were "wireless" communications during WWI, but these were in Morse Code, not voice, and were between sending and receiving radio installations. No one had home receivers to listen to this stream of dots and dashes. Marconi is credited with discovering the principals of "wireless telegraphy", which was in about 1908.