In this clip from Cosmos, Carl Sagan breaks down the Drake Equation - a formula that uses a string of variables to determine the probability (and quantity) of detectable extraterrestrial civilizations in our galaxy and the Universe.
The Drake Equation:
N = the number of civilizations in our galaxy with which communication might be possible;
R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space.