78 (72 active and 6 under construction)
There are two different types of submarines...
The Attack Sub, which is designed to seek & destroy enemy subs & surface ships.
The Ballistic Missile Sub, which is a nuclear-powered sub armed with long-range strategic missles.
As for as Attack Sub's, there is 1 Virginia Class (5 under construction), 2 Seawolf Class (1 under construction), & 51 Los Angelos Class.
As for as the Ballistic Missile Sub there are currently 18 active