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)
From element number one, hydrogen, to element one-hundred eighteen, ununoctium, there are 118 confirmed elements on the Periodic Table. Ninety-four of the elements exist in nature, and the remaining twenty-four have only been synthesized in labs (not having occurred naturally in the time since scientists have begun researching them)