In the early 1960’s Frank Drake sought a way to calculate the probability of finding other intelligent races. He came up with an equation to calculate the possibility of extra terrestrial civilizations; which came to be known as the Drake Equation. Using this equation Mr. Drake determined that there was the possibility of 100,000 to 1,000,000 extra terrestrial civilizations in our Milky Way Galaxy.
N=R*FpNpFlFiFcL 

N = the number of technological civilizations in our galaxy R* = The rate of formation of suitable stars F(p) = the fraction of stars having planets. N(e) = the number of suitable planets per planetary system. F(l) = the fraction of planets on which life develops F(i) = the fraction of life that evolves to intelligence beings. F(c) = the fraction of intelligent species to develop the means of communication. L = The length of time such civilizations release detectable signals into space. 

Drake and other early extraterrestriallife enthusiasts, including Carl Sagan, arrived at a value for N between 100,000 and 1,000,000. This is roughly about one technological civilization per million stars in the Milky Way, which astronomers believe contains some 200 billion stars.