Chicago recommendations on possessive apostrophes runs four pages with a couple dozen rules and exceptions.
Generally, how a possessive term is spoken is a guiding principle. Number in some cases has no bearing.
Marquis', singular (unpronounced S in the uninflected form); marquises', plural.
People's, uncounted plural.
Children's, uncounted plural, (childrens'es!? an inflected dialect form. Yeah, I've seen it in a transcript of spoken word).
Susan St. James', apostrophe appended to a singular name, James.
The James's, a plural possessive for Mr. and Mrs. James and, if any, their kin. However, optionally, with context definitively indicating plural possessive, The James' car broke down.
James' books, but optionally, James's collection, for number agreement
Grand Prix' cars, but razzmatazz's glitter.
A church's minister, churches' congregations.
But in titles or other situations where there's clearly no possessive context;
The Smiths Tavern
Signmakers once upon a time routinely omitted apostrophes according to their styles. Probably as much a consequence of a pleasing appearance and economy of effort as anything. How does an inanimate object possess itself? is the debate on whether to include an apostrophe in a business' name, for example. Merriam Webster followed suit in early dictionaries' style recommendations. That style recommendation has largely gone by the wayside, though, of late.
In formal writing, whether to add a possessive S to a singular or plural word is not a clear-cut prescriptive absolute.
[This message has been edited by extrinsic (edited July 29, 2009).]