Bluebulb Projects' The Measure of Things presents:
21,000 is about 200 times the number of Elements on the Periodic Table.
In other words, it's 178 times the count of Elements on the Periodic Table, and the count of Elements on the Periodic Table is 0.00562 times that amount.
(a.k.a. Periodic Table of the Elements, a.k.a chemical elements) (confirmed elements as of 2016)