The U.S. navy currently has 22 active classes of ships.
Don't get me wrong, that's a good point ...
World wide there are over 3,000 active naval vessels, which include ....
20 aircrat carriers of 10 different classes.
169 destroyers of 32 different classes.
422 Frigates of 49 different classes.
... but at the same time they aren't building four or five different types of attack subs ...
Of the 398 attack subs in active service, 59 subs of 4 classes came from America (43 Los Angeles class, 2 Seawolf class, 6 Trafalgar class, 8 Virginia class).
5 different active service attack sub classes were built in Germany, 6 in Russia.
It occurs to me that all
the Nebula class ships might be precursors of the Galaxy class, the Galaxys are a follow on class, with the Nebula construction being discontinued prior to the construct of the first Galaxy.