Strange things occasionally cross my mind, for instance: If one rabbit has a carrot, it’s the rabbit’s carrot. If multiple rabbits have carrots, they are the rabbits’ carrots. If a single deer has a carrot, it’s the deer’s carrots. But the plural of deer is also deer, so what’s the plural possessive of deer? Would it be deers’ carrots? Doesn’t look or sound right.
It is "the deer's carrots," no matter how many deer are there. Every style manual seems to have a different rule about this, and you'll have to bow to whichever manual your publisher considers gospel. My own rule of thumb has always been that if the possessive S is actually pronounced, it deserves to be there. otherwise, out it goes. This is is where the Smiths live. It is the Smiths' house. Ms. Smith's car is a Ford. And here is where the Joneses live. It is the Joneses' house. Mr. Jones's car is a Toyota. And I'm a bit of heretic by writing "Jesus's teachings" and "Moses's leadership," when many style guides insist that it's "Jesus' teachings" and "Moses' leadership." But my take on it seems to be gaining favor slightly.
English is the most difficult language because there are no rules that don't have a multitude of exceptions.
