The Drake Equation attempts to answer that question. There are 200-400 million stars in the Milky Way Galaxy and there are hundreds of millions of galaxies in the universe.
N is the number of civilizations in the Milky Way Galaxy.
This number is derived by multiplying the following factors:
R is the rate of star formation
fp is the fraction of stars that created planets
ne is the number of Earth-like planets
fl is the fraction of planets that actually have life
fi is the fraction of planets that actually have intelligent life.
fc is the fraction of planets that actually have interstellar communication.
L is the amount of time this civilization remains detectable
Drake got 12,000 when he solve this equation As Drakle was quoted as saying, "That's a lot of E.T.'s"