20000 was a number.
The Malamud noted that, because of the many volumes about the Doctor's life, he must have been 20,000 years old. The Sixth Doctor did not contradict them. (PROSE: The Book of My Life)
The Twelfth Cyber Legion was situated 20,000 light-years from Demons Run when it was destroyed by the Eleventh Doctor. (TV: A Good Man Goes to War)