[FroshKiller]

The plural would regularly be Hopkinses in English. If I invited your family over, I would say the Hopkinses are coming. If I was going to your family's house, I would say I'm going to the Hopkinses' house. If you specifically were giving me a ride in your car, I'd say we're taking Don Hopkins's car.

It's not a big mystery or even particularly complicated, all regular rules of pluralization and the possessive case. I think people get tripped up in school because they see a specific affectation of dropping the S from possessive forms of the names of some historical personages, e.g. "in Jesus' name."

[lsaferite]

Seeing "Hopkins's" is very weird to me. I was taught that the possessive of a noun ending in "s" just got a trailing apostrophe. Is that no longer the norm?

Edit: As a partially related aside, I have a friend who's right about the same age as me that's incredulous that I was taught to use "they" as a gender neutral pronoun when the subject's gender was unknown (or you desired not convey a gender) back in the late 80s and early 90s. Maybe it's just a regional difference in teaching or something. He's from the UP of Michigan, I'm from Florida. So maybe the same thing is true with possessive nouns

[tim333]

In practice there seems to be some variation in how people write it. Wikipedia has "List of Anthony Hopkins performances" Guardian has "Anthony Hopkins' 20 best film performances – ranked!". The s's seems rarer but the Irish Times has "Anthony Hopkins's new prestige".

[bagful]

In the former case it is a list of performances characterized by including Anthony Hopkins; in the middle, it is a list of performances belonging to Hopkins. The latter reads incorrectly to me as an American but as a possessive nonetheless.