So you've read a Song of Ice and Fire. You see George RR Martin with his million POVs and you say "Oh man that rules! I gotta do a big, huge, multi-POV story! It'll span a dozen books and be so super cool and epic and everyone's story will be awesome!"
It turns out, that is really really hard to make good. So hard that seriously experienced authors routinely let their stories get away from them. GRRM has even been trying to control his story's spread by slaughtering his characters left and right. He does this, and books 3 and 4 STILL had to be separated, even though they were essentially one gigantic novel, with all the events occurring at the same time. And this is a man with decades of experience writing. He is very familiar with the writer's tools. And he still has problems with spread.
Steven Erikson has a problem with this spread as well. His Malazan books are delightfully written, very vivid and full of gore, but by the time we got to book 6 in the series we're reading an entire novel that's just the backstory for a side character that we just met in book 5. I'm not Steven Erikson, but that sounds like an issue where his story got so gigantic and convoluted that he had to sit down and write something else to give him the time to actually settle his main plot. OR he just wanted to write this other story and it's in the same universe so why not call it book 6?
This is a pretty easy issue to visualize, if you look at your character's plotlines like colored strings. If you have two or three, or even four, there'll be some places where they get knotted up, other places where one loops away and goes somewhere else but eventually returns, but you're always able to keep track of them. When you start your book with 9 POVs, then add interesting side characters as new POVs in later books, you're ending up with a titanic morass of jumbled strings where you have almost no ability to tell one from the other.
There's a lot of discussion as to "what makes a book." Like, the actual thing. Where does it start, and where does it end? Most authors would say that a book ends when you have a solid sense of resolution. Not cleaning up everything - that's for sequels, but there has to be a conflict that is either introduced or carried over, and it has to get resolved. It's what we refer to as "story arcs." When you have books with a million characters, you have to write a ten million word book just to get those characters to all move ten feet. Is that really a book? Was there conflict, and was it resolved? Or was this "novel" just a gigantic set-up for the actual novel.
There is an actual example of this, in a series I haven't mentioned yet but most will at least recognize: The Wheel of Time. If you haven't heard of it, it is a FOURTEEN BOOK series, with each of the novels being something like six hundred thousand words. They are HUGE. Book ten in the series is a book called Crossroads of Twilight. In it, the characters decide where to go and what to do. That is the book. It was far, far too difficult with that many characters (there are over a dozen at this point I think) to introduce a story element and have each of them react to it in a way that moved the story forward quickly that didn't seem disingenuous. These characters all had different ideas of what to do, how to do it, and different plots in the world moving to stop them. I'm going to spoil something for you here, but in the Crossroads of Twilight, everyone decides what to do. The actual novel is book eleven, Knife of Dreams. There, all the characters have set up what they're doing, gone to their various places and decided who to kill.
Epic stories are awesome. There's no feeling more amazing (to me) of seeing all these incredible characters and convoluted plots finally resolve into one epic climactic thing. But so often, that doesn't even happen. The plots have gotten so convoluted that you can't even find the end of your metaphorical strings. The characters have become so different and independent that they don't even want to deal with each other. There's so many characters and plots that even when you do slam everything together in a climactic finale, there's too many loose ends for it to actually feel resolved.
This is one of the reasons that when Brandon Sanderson came in to finish WoT, Jordan's notes for the final book were so extensive that there was literally no way to pin it down all in one book. SO HE MADE THREE. The finale of a series, in an epic trilogy. That's kind of awesome, but at the same time, insane that it's required.
It's not always multi-POV stories that get too epic and too crazy. Did anyone read Xenocide and Children of the Mind? Those were completely bonkers. That might just be Orson Scott Card being a little cuckoo, but it's what happens when your characters reach the apex of their skill and might and you still try to tell stories with them. If your MC is an uber-god that can't die and destroys planets with a thought, you are going to have to come up with some crazy ass shit to make conflict! Or, be lame and wipe their memories or something (lookin' at you, Heroes)
So it's important for you to decide. How epic can you be, realistically? With your own talents and your characters and your world, how epic can you actually be and keep the story tight and still spellbinding? When you have all these characters and plotlines, can you still write a book that someone can just come and pick up and read? Or do you need an army of assistants and a full wiki to keep everything straight for yourself, let alone a new reader.
I'm not telling you to not write an epic multi-POV story. That's what I'm writing! But I am suggesting that you keep the number of those POVs small, at least to start. Make sure you can actually write real books, not just a collected binding of chapters that had to be separated from the real story arc because booksellers wont publish 2000 page novels and also no one will buy those. Keep it tight. Keep it awesome. Keep it epic.
But not too epic.