5000000000 was a number.
The Ninth Doctor approximated the year 5.5/Apple/26, also known as "the year five billion", as five billion years in the future from Rose Tyler's native time of 2005. (TV: The End of the World, New Earth)
According to the Ninth Doctor, the population of Earth in the year 2006 was five billion. (TV: World War Three) It would later jump by an entire billion within two years. (TV: The Eleventh Hour)