Bluebulb Projects' The Measure of Things presents:
50 is about seven-tenths the number of Submarines in the US Navy
In other words, it's 0.690 times the count of Submarines in the US Navy, and the count of Submarines in the US Navy is 1.40 times that amount.
(a.k.a. Subs) (includes all types, all classes) (2015 figures)