Drake's equation provides a way to quanitatively estimate the number of civilizations present in our Galaxy that are are technically able to communicate.

N = R*fp*ne*fl*fi*fc*L

N = The number of communicating civilizations is the product of the following:
R = The star formation rate in our galaxy: the number of new stars formed per year. **
fp = The fraction of stars with planets around them.
ne = The number of habitable planets per star system.
fl = The fraction of habitable planets that develop life.
fi = The fraction of living planets that develop intelligent life.
fc = The fraction of intelligent species that develop technology capable of communicating across space (e.g., radio broadcasts).***
L = The lifetime a civilization spends in the technological broadcast phase.

** We might wish to restrict this to solar type stars or otherwise consider only "suitable" stars.

*** This implies a willingness to make the effort to use technology to communicate. We've been broadcasting I Love Lucy for decades, but the signal will quickly fade thanks to the inverse square law. Even if another species can detect this broadcast from many light-years away, would this constitute a recognizable attempt at intelligent communication?