Well, on some occasion plurals of names are used:
Your Mom has a lot of Johns.

Although, in this case, it's not a proper noun any more but rather a synonyme to "customer".
On some ship classes it's easy to form the plural because the ship class is not a proper noun. Names like Deimos, Hercules and Hecate are all proper nouns (names of unique things), whereas Valkyrie, Manticore, Basilisk and some others already have a plural form because they are names of groups of things.
But it would indeed be kinda stupid to talk about how many Nimitzes the US Navy has...
