Bluebulb Projects' The Measure of Things presents:
21,000 is about 4,000 times the number of Living U.S. Presidents.
In other words, the count of Living U.S. Presidents is 0.0003 times that amount.
(a.k.a. Presidents of the United States; a.k.a. Presidents of the United States of America; a.k.a. POTUS), (2016 figures) (including current)