Bluebulb Projects' The Measure of Things presents:
66 is about three-fifths the number of Elements on the Periodic Table.
In other words, it's 0.5590 times the count of Elements on the Periodic Table, and the count of Elements on the Periodic Table is 1.790 times that amount.
(a.k.a. Periodic Table of the Elements, a.k.a chemical elements) (confirmed elements as of 2016)