Answer: At Least 4 Billion
Carl Sagan was an American cosmologist, astronomer, and absolute tireless champion of the sciences in the public sphere. He was the author, co-editor, or editor of almost two dozen science books, and the host the award-winning 1980 television series Cosmos. Sagan was well known for his excitement in talking about science, especially cosmological issues, and would strongly enunciate the M sound in millions and the B sound in billions to emphasize just how big the numbers were and properly differentiate them for the audience.
Between his strong enunciation, a chapter in one of his popular books called “Billions and Billions”, and a Johnny Carson skit parodying Sagan’s love of the billions and billions of stars out there, the public came to associate Sagan with the phrase. In light-hearted tribute to Sagan, fans of his work and his tireless drive to bring science into the public eye, began using his surname, Sagan, as a unit of measurement. A Sagan signifies a quantity over four billion because, at minimum, billions and billions would have to be at least two plus two billion.