Q. What is the plural of “Mercedes”? For example: “The armored Mercedes’ of the oligarchs sped through the streets of Moscow.” “Mercedeses” sounds clunky, but does the final apostrophe adequately convey the plural?
A. An apostrophe can sometimes signal a plural, but it does that only in combination with an s (e.g., three x’s). Mercedes’ doesn’t read as plural.
And though a proper name ending in a pronounced s normally forms the plural by adding es—for example, a family with the surname Jones would be known as the Joneses—we wouldn’t recommend that approach in this case.
A written invitation to lunch at “The Mercedeses” might be strictly correct (for the surname Mercedes), but as you suggest, that would be awkward to pronounce (and just as difficult to read). For the car, allow Mercedes to do double duty as both singular and plural.
One caveat: Unless context makes it clear that Mercedes is being used as a plural, you may have to clarify—for example, by adding a collective noun: The oligarchs’ armored fleet of Mercedes sped through the streets of Moscow.