Find you someone who can do both...kinds of act structure.
The Hunger Games. Harry Potter. Twilight. Percy Jackson. The list of book series studied for act structure has long been discussed on the internet. The reality is that all of these series execute act structure well. They sustain engaged readers. The story gets told. We can study act structure, read all the blogs and course through all the books on it--which I encourage that you do so--but there's one thing that isn't discussed enough when it comes to act structure in books series. What about your novel series? What kind of act structure does your novel need in order for your story to be engaging, consistent, and satisfying?
The answer is actually easy. Use whatever act structure you want it to have.
There's no doubt that studying act structure is imperative. Whenever I'm working on a new project, I revisit a handful of books and articles at various points of the writing process. As I get deeper into the manuscript, and even further into the revision process, I find that my structure changes over the course from writing the first words of my outline to the putting in the final line in the last chapter in the first draft. For me, when I'm done that first draft, I revisit my outline (which also changes a few times over the course of first draft), and then start re-writing/revising a second draft. When I get to the end of the second draft, I have a much clearer idea of what sticks, what darlings have to be killed, and where my story has to go in the third draft (yes, I draft each book three times, twice before it gets to my editor) and where I want the story to go heading into the second novel.
To put it simply: you will learn what your act structure will be as you write.
This may sound like I'm saying go in blind. I'm not. Outlining is important and I suggest you spend a considerable amount of time thinking about what you want to happen in your story and how that ties into the themes. But don't marry your novel to the outline at first. Sticking to a strict outline might leave you at odds when you realize your story has some plot holes, or your characters (or even you) have changed over the course of writing that first draft.
When it comes to writing a novel series, determining how to structure your acts largely depends on a couple of factors.
Remember in the beginning when they were just a bunch of naive lil' munchkins?
The Harry Potter series has a three act structure that extends over the course of seven books. Rowling breaks these three acts down and pairs up the novels. The first two entries in the series act as the first act of the overall arc, but each book has its own stand-alone three act structure as well. If you do the math, Harry Potter has twenty-one acts in the series, not including the three overarching acts.
What's important is that you come to a clear understanding of what act structure is. Write to your heart's content. Outline, write, revise, study, repeat. The structure will find its way into your plot as you continue to learn about your story, research the rules of act structure, learn how it applies to your work, and then figure out how to bend or break those rules in order to sculpt it into what works for your manuscript.
There's no magic bullet when it comes to figuring out the best way to structure your novel. The answer will come eventually, but trial and error have to be part of the process beforehand. No one can tell you what works but you.
...and your editor, who will eventually look over all your work with a meticulous, caring red pen.
The answer is indeed easy. The work, however, is not.