Frank Drake, President of SETI ( Search for Extraterrestrial Intelligence), following his observation of Tau Ceti and Epsilon Eridani stars under Project Ozma, has calculated his famous equation which can be used to formulize the number of the intelligent civilization in Milky Way Galaxy.
The equation is N=N*.fp.ne..fl.fi.fc.fL.
N represents the number of the intelligent civilization in Milky Way. N* represents the number of the stars in the galaxy. While fp is the number of the stars which have planets in their orbit. "ne" is the planets which can support life. "fl" is the number of the planets in which a life form or structure is evolved in "ne" and "fi" is the ratio of intelligent life forms in "ne". "fc" is the number of the civilizations in "fi" which are able to contact with others. And lastly fL represent the lfe duration of the civilization on the planet which can contact with other solar systems.
According to this formula he calculates the number of the intelligent civilizations which are able to contact with others as N= 2.400.