Quest for Character Lists

I am very interested in how novelists introduce and maintain characters within a large body of work. I say large body of work referring to multiple novels in which the characters appear multiple times. Well known examples: J. K. Rowling's Harry Potter series, J. R. R. Tolkien's body of work.

A cursory scan of Rowling's books reveal at least 700 characters who are referred to in some fashion over 7 novels and two "reference" books. There's a Wikipedia Tolkien list for Middle Earth characters which says it is an incomplete list--but it lists around 560.

My next goal is to go through Jasper Fforde's novels--both the Thursday Next series and the Nursery Crimes series. Those are fun novels--with lots of familiar fictional characters from the literary cannon. There's got to be quite a large list there...

What drives the need for such a large cast? Why mention characters just one time or obliquely refer to them?

As a reader, I enjoy the breadth it gives a novel. There are threads that can be hinted at in one place and picked up at a later part of the book or even in a different book. As a writer, I would imagine it gives wiggle room and provides alternate possibilities when something isn't "working" with a first idea.

Yes, this is a curious way to approach literary study. It is also most likely NOT the way to approach novel writing. But I've never been a "paint by numbers" kind of girl--I will do it my way. If my way doesn't work, at least I've learned one way to not accomplish the goal.

Anyway, back to my quest...