Bluebulb Projects' The Measure of Things presents:
21,000 is about 300 times the number of Submarines in the US Navy.
In other words, it's 290 times the count of Submarines in the US Navy, and the count of Submarines in the US Navy is 0.0034 times that amount.
(a.k.a. Subs) (includes all types, all classes) (2015 figures)