In three half-lives the C14 concentration decreases by a factor of 8.
Twenty thousand years is 3.49 half-lives of C14 because 3.49 times 5730 is 20,000.
The half-life of C14 is 5730 years, although there are subtleties about how C14 ages are actually computed.
That means that in 5730 years, half of the C14 will decay to nitrogen 14.
The half-life of Carbon $14$, that is, the time required for half of the Carbon $14$ in a sample to decay, is variable: not every Carbon $14$ specimen has exactly the same half life.