I'm planning a book series called The Weasel Sagas. Things have been going great up until now, where I've realized I've hit a bit of a problem: I've come up with backstories for so many different characters all over the hero-villain spectrum that it would take 50 books to have time for all of the characters' motivations, backstories, character development, and arc resolution along with the plot and huge mass of worldbuilding. I've only planned 28 books for the entire series, and I don't think I'd live long enough to write 50 books. How would you suggest I tackle all of these characters while keeping the number books at 28 and needing as little simplification as possible to keep the story compelling?

Just so you can understand the sheer scale of this project, here are the three groups of MAIN characters:

Heroes (Knights of the Square Table): https://sites.google.com/site/weaselworldofficialsite/characters/the-knights-of-the-square-table

Villains (The Cabal): https://sites.google.com/site/weaselworldofficialsite/characters/the-cabal

Supernatural Characters (Deities, Spirits, Etc.): https://sites.google.com/site/weaselworldofficialsite/system/app/pages/subPages?path=/the-metaphysical

I agree largely with Jedediah's answer, and want to add something else to it. I'm also going to preface this with saying I've glanced through a couple of the links provided, but didn't bother reading more.

First off - what you've listed aren't Main Characters. You've listed the players involved.

There is a staggering difference between Main Characters and supporting characters. Take, for example, the Gaunt's Ghosts series by Dan Abnett. It follows the story of an entire regiment of soldiers across a war-torn galaxy. There is, arguably, one main character (the titular Gaunt) and a whole swathe of supporting characters, on both sides of the conflict of differing importance. We know them, we recognize them as distinct, but they aren't the main character. Whether it's the warmaster himself, or the arch-nemesis - they are named, given life but aren't main characters. They don't need a backstory beyond what the story itself demands.

Likewise, the biggest reason not to include it, is no one cares. The reader does not care that the main characters trusty companion once had his ice-cream stolen by a goose and his mothers favourite colour was cyan - unless it is directly related to the current story.

In the ever-so brief readthrough of your character backstory, I can't remember a single interesting, relevant fact. It's great for YOU to have these notes, as it helps YOU, as the author, build the characters - but it is entirely irrelevant for the reader.

All the reader cares about and needs to know is that the Knight's of the Round Table are a brotherhood of knights opposed to the secretive, evil Cabal. Unless it brings some kind of personal conflict to the current story, then we don't need to know why each individual is opposed (I'm guessing there were a lot of lost heirs/burned villages/slaughtered family etc) just that they are, collectively, opposed.

Likewise, we don't need to know the backstory of why the villains are doing what they are doing, or some unrelated campaign 10 years ago.

So in summary, make as many notes as you need to give the characters life. But keep them for yourself - they don't need to be in the story itself - unless it is directly relevant to the story currently being told.

It is neither necessary nor desirable to fit everything you've generated for a story into the story

In my reading I have encountered, broadly speaking, two different kinds of stories. There are tightly-plotted stories which attempt to resolve and give closure to every thread the author introduces. (The Westing Game comes to mind.) This is great.

On the other hand, there are stories with incredibly sophisticated backgrounds (like Lord of the Rings), where the author has volumes of supporting material which could not fit into the actual story. This is also great.

I suspect that, even in the tightly-plotted cases, a good author may have material which did not directly show up.

In the real world, there are endless subtle influences on your behavior which need not be explicitly included in a story, even if their unseen mass affects the orbital paths of those characters featured. I don't need to tell you how the size of my family as I grew up, or the relative wealth of my family, still influences my decisions at the grocery store - although knowing what kind of situation your character grew up in will help you generate a more interesting "shopping list," or whatever is needed in your story.

Let your larger universe add hidden texture to the few characters that fit into each tale, and don't worry that there isn't time or room to fit everything in right now

You don't. You reduce the importance of some characters to reinforce the importance of the ones critical to the plot.

Do you remember hearing a fairy tale as a 5-minute bedtime story in your childhood, then seeing Disney make it a 90-minute musical? How did they do that? Well, in many cases that story has a novel-length original source. For example, de Villeneuve gave us Beauty and the Beast in 1740, and it was quite long, only to be gradually trimmed to the version you heard before you went to sleep

What does this have to do with your question? I want you to imagine how dear Villeneuve wrote the original. Presumably, she had a rough story idea about as detailed as the shortest version you've heard, then gradually wrote it. But here's the crucial detail: when you have enough of an idea to write a novel, you don't have to write a novel. Oftentimes what you should really do is keep certain things in your head, then draw on them when working on what really deserves a novel-length treatment. Hemingway likened it to most of an iceberg being invisibly submerged; most of the 50 novels' worth of ideas you have in your head should never, ever be such novels.

I'm sure you want an example of something that does it well. I recently rewatched the first two seasons of A Certain Magical Index because the third is airing now, and I also watched the two seasons of its spinoff, A Certain Scientific Railgun . It's a franchise with a huge cast of characters, and you very quickly get inside the heads of all of them. But only three of them, Touma, Misaka and someone known only by the codename Accelerator, have been protagonists in the franchise, and that last one doesn't have his own series; he's just the guest protagonist in several Index episodes. Index is one of the characters, and she doesn't even get to lead what's named after her! So the author has had to be very selective about what stories are told and from which perspectives. I can't recommend it enough add an exercise in great writing for many reasons, including as building a world and its characters succinctly.

One of its tactics is a protagonist unexpectedly encountering new people and having to quickly learn and react. Another is to experiment with various characters interacting in new combinations in side scenes, allowing several to be developed simultaneously.

Look for models of stories with a colossal cast you can use as a model for your series. Many books use the device of a story within a story in order to go off on tangents.

One example of a book that uses this well is Astrea. It is a long text with a long plot and 273 (!) characters, but it manages to follow one or two main plotlines for the most part, while the exploits of the other characters are often relegated to stories which are narrated by the characters of the main plot.

The outline of the text might look something like this:

Chapter 1

(Main Plot)

Chapter 2

(Main Plot)

Story

(Side Plot)

Chapter 3

(Main Plot)

Clearly marking which sections are essential can help avoid making your reader confused. If there are sections which are of interest for reasons of character but not for plot (i.e., that the reader can skip without missing out on plot development), they should be marked as such so as not to leave your readers bored if they are not interested in those stories.

The issue with having a "colossal cast of characters" is simply that there is only a finite set of unique traits that you can give a character. Having too many might let your characters overlap in characteristics and motivations, which would ultimately diminish their worth in your story.
Additionally, by having less characters, you give more room for their personalities to juxtapose, or act as foils to their fellow characters. Having a huge quantity of characters, while appealing at first, ultimately wouldn't work as a first book, as you either wouldn't have time to flesh the characters out individually, or would have to rush through them, both of which would negate the reader's perception of your novel.