So I would highly recommend you look at K.A. Applegate's Animorphs series of books. The main series contains 54 books with 4 additional Megamorphs titles (featuring the Main characters but had little to no impact on the main title books... 3 of the four featured events that were undone by the end of their story) and 4 "Chronicles" novels which were pretty much prequel events to the main title. All books are written in First Person Prospective though the narrative character will change. Incidentally they had
The main book titles featured one of the six characters (Jake, Rachel, Tobias, Cassie, Marco, and Ax) doing the narration, with some exceptions (Books 19, 34, 42, 47, and 54). These books would also follow an ordered rotation of narrators that was changed late in the series. From books 1-40 (inclusive) the rotation was ordered such that each character got two books out of every ten, except Tobias and Ax (who got one book a piece out of every 10, due to being notoriously difficult to write). This saw that each character would consistently get books that ended in the following digits:
Jake (1, 6)
Rachel (2, 7)
Cassie (4, 9)
Marco (5, 0)
Starting on book 41 on, The rotation followed that the narrators would cycle Jake, Rachel, Tobias, Cassie, Marco, Ax so that the full rotation occurred every six books.
The Megamorphs Titles were generally seen as not having much impact and were basically a few breathers before or after some major plot element. The big difference of the Megamorphs from the main title was that all books were narrated by all six characters over the course of the story, though it was as needed and not as in any particular order. Three of the four dealt with time travel in someway, so while the events are remembered by the characters, the events got hit with a reset button and undone at the end. And two highlight that these events were going to be reset, at least two books had a character die and thus, was not narrator until the death was undone... if at all for that book.
At the time they were written, main title books were written once a month, so the Megamorphs usually served to delay the release of the major event for another month... or serve to introduce a narrative device that the next book relied on (The first was after a major victory for the heroes in book 7 and just before the first book Ax would narrate in full, allowing readers to get a feel for his POV. The second one was right book was just after another major victory in book 18 and just before a very emotional book 19 which is the first one to feature two narrators (possibly placed to emphasize the way the switch worked). The third occurred before book 30, which was another emotionally heavy story and the fourth occurred before the new rotation was introduced.).
Finally, the Chronicles Books normally followed the POV of a secondary character in full, and deals with issues that occurred prior to the events of Book one, though all of them are framed as the protaganist recounting memories during some point in the main line series (The novel "Visser" does not have a Chronicles title, but it's structurally the same thing... it also is the only book to pick up directly from events of the previous main series). The Hork-Bajir Chronicles is the only Cronicles that featuse multiple First Person narrators (four of them) and, which follows the Megamorph format.
As the books had numbered chapters with no titles, any time a narrative transition occurred, it was always during a chapter break. The new narrator would be named (The Chapter would read "Chapter 6: Jake" and followed by a picture of the narrator, normally taken from the cover art or recent book with the character) and the next chapter would feature that chapter's narrator, even if it was the same character (though this was rare).
In the Main Line books that did this transition, the character who was the narrator in that rotation would not be introduced in every chapter in this format, but rather, only when the guest narrator was transitioned out and the regular would resume their narrative. The only exception to this was book 54, as it was the last story in the series, so it took turns describing everyone's reaction to the aftermath, though it did start with the proper character in the ordinary rotation (Rachel).
I know it's a lot to describe, but this was mostly to show that the system was largely consistent and was used to great effect through out the series.
/* The reason for this was that both characters were very hard to right consistent stories like the other four. In the case of Tobias (who had been trapped as a Hawk morph in the first book), he filled the team's scout role and spent significant time alone and isolated from the other characters. Most of his books were very introspectively narrated or required him to go on solo missions without much in the way of back-up. Ax, being an alien among five humans, was also featured in stories that were mostly him having long solo moments and often focused on him being put into moral dilema's that forced him to choose between his loyalties to his Honorable Warrior Race and his Human allies that he worked with, specifically Jake, who Ax early on professed an oath to serve (Honorable Warrior thing...) but realized Jake was not always the most orthodox leader by his alien standards. He would often keep information from the rest of the team because of this and had to work his way through the ramifications of those choices.