I would first point out that beginning writers with no commercial success will approximately never sell a multi-book series.
The rule for a multiple-book story is relatively simple: The first book must absolutely stand alone. And every book must absolutely come to a conclusion that is satisfying even if it turns out to be the final book in the series.
That is not necessary true for commercially successful authors; Stephen King could likely publish a trilogy that doesn't meet these rules, because his reputation and following of paying customers ensures all costs will be recouped almost no matter what.
But if you look at the most successful series of all time, Harry Potter, JK Rowling was an unknown author, and the first book and following books very much stand alone. She does introduce more world elements as she goes along, that's fine.
That leads us to your epic, of 2 or 3 or 4 books.
How many independent novel length stories can you craft out of that?
What can you do in 100,000 words that can be an independent novel, with all the story arcs, following the Three Act Structure (3AS), and not leave the reader disappointed and wondering how the story ends?
The 3AS is recursive; the trilogy as a whole basically follows it (or a quadology?), The first book details a major problem, the 2nd (and maybe 3rd) develop complications, the final act resolves everything.
But each book must also follow that structure; Problem, Complications, Resolution.
At the end of the first book, the initial problem is solved, the world is introduced, but the bigger problem is just understood, not resolved.
At the end of the first star wars movie, the Death Star is destroyed but Darth Vader is not dead. In the second movie, Vader returns and the rebels are on the run. In the third, the rebels prevail.
You must do the same, each book must serve two purposes simultaneously: It must serve as an Act of the overall 3AS story, while simultaneously being a self-contained 3AS within its own 100,000 words.
That's how the business works.
Once you are successful, you may have more leeway. But even JK Rowling, the most financially successful author ever, wrote a series with each book having its own 3AS. Readers could be confident that buying the next book in the series, while developing their favorite characters further, would introduce a new problem, then complications, then have a satisfying resolution of the problem introduced.
So this is how your "how many books" question is resolved. It is not really about how much information and education you need to do. It is about how many independent, 100K word 3AS stories you can develop. If you have 400K words to write, you need 4 or 5 consecutive but independent 3AS books.
I'd suggest for the first one, you steal the approach used in Star Wars: The first film (we the audience see) acts as the "First Super Act" by introducing our hero, Luke Skywalker, and moving him from novice farmboy to Jedi Knight, an accomplished warrior. Also while introducing the world to the audience for the first time. (Edit: Also, Luke has firmly left behind the "farmboy normal world" where he began; as we'd expect from a First Act. He's a rebel soldier now.)
At the end of the first Super Act Luke is not a master, he's not ready to fight the big evil (Darth) directly, but Luke delivers on a key battle in a bigger war; he destroys the Death Star and puts Darth on his heels; spinning off into empty space. The Death Star is basically "the inciting incident" for the trilogy.
I imagine you need a similar transformation of your hero.