Within the limits of our existing technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. In each of its last four decadal reviews, the National Research Council has emphasized the relevance and importance of searching for evidence of the electromagnetic signature of distant civilizations. -- SETI Institute
The Drake Equation, developed by Frank Drake, which he presented in 1961, serves as a benchmark formula to estimate the number of likely intelligent civilizations that might be out there. For those who are mathematically inclined the equation is in this footnote.
Having written Broca’s Brain in 1974, Carl Sagan would have been aware of the equation, and I assume his remarks in that book about the calculated figure are based on it:
When we do the arithmetic, the sorts of numbers we come up with are, characteristically, around a million technical civilizations [in our galaxy]. A million civilizations is a breathtakingly large number, and it is exhilarating to imagine the diversity, lifestyles, and commerce of those million worlds. But the Milky Way Galaxy contains some 250 billion stars, and even with a million civilizations, less than one star in 200,000 would have a planet inhabited by an advanced civilization. -- Broca’s Brain, p 315.
How many planets exists that might support life? Indeed, what is required for life to exist? How does life start? How does it evolve, and what fabulous creatures can evolution produce? How often do intelligent creatures appear in the giant tapestry of life? It is exactly these questions that are being addressed by the scientists of the Carl Sagan Center for the Study of Life in the Universe.