We have 42 active Los Angles class hunter-killer submarines

[HooptieWagon]

10. By my rough count,

the US had approximately 110 fleet subs of various models in service at the time of Pearl Harbor. These were built between 1918 and 1941. Another 215 were built during the war, primarily of the Gato class. Some of the earlier subs were used for training when the Gatos came in to service, some were transfered to allies, some remained in service untill the end of the war. It appears the Navy discontinued use of all the pre-WW2 subs at the ending of the war.
Of the Gato class boats, it appears some continued in commission by the US Navy until the 70s (but perhaps in mothballs?), some that were sold to other countries appear to have been in use until as recent as 1990.
The Los Angeles class boats can remain at sea far longer than even the Gato boats, maybe 1 LA = 2 diesel/electric subs? So, maybe somewhere around 55-60 Los Angeles boats might be a desireable number, in my estimation.