We have 42 active Los Angles class hunter-killer submarines

[Lurks Often]

15. There are 52 attack subs in active service

The 42 Los Angeles, the 3 Seawolf and the 7 Virginia class, which is the class that is replacing the Los Angeles class.

Those are split between the Pacific, Indian & Atlantic Oceans (I include the Arctic Ocean and the Mediterranean Sea as part of the Atlantic, since subs deployed to these 2 areas would come from the Atlantic Fleet). Of the subs, probably only 30-40% are ever at sea at once, with the remainder either: training up for the next deployment, performing post-deployment activities or in some form of routine maintenance or overhaul.

In other words, we have only 16-21 attack submarines at sea to cover the Pacific, Indian & Atlantic Oceans, really not that much when you break the numbers down.

On edit: See http://www.globalsecurity.org/military/ops/where.htm

I based my percentages of submarines deployed as being roughly the same as the carriers and their escorts.