How many characters in a story?
Let's say you have 90K words to play around with. In that space, you can develop between seven and ten characters. A dozen at the outside.
Three or four can be major actors with flaws and inconsistencies and pasts. With POVs. Another six or seven can fill the role of secondaries. We see a good bit of these secondary guys.
But we end up with just these two or three cool main characters.
In all -- a dozen recognizable and developed characters.
Not two dozen. Not twenty. Not even seventeen.
There's a reason movies are named 'Ocean's Eleven' or 'The Dirty Dozen', instead of Ocean's Twenty-two and The Dirty Thirty. The Guns of Navarone was a team of seven plus a villain. How many characters perform significant action in The Great Escape?
(And that was deliberately plotted for cameos.)
If you have nothing to do some rainy weekend, you might watch these pictures and study how the screen time is allotted. It's a good lesson in handling multicharacter fiction with de-emphasized main characters.
In these movies, we do not have seventeen developed characters to keep track of. When you look carefully, it's more like eight.
There is the Mother Cat Effect.
which I have just named.
I am not advocating the theft of kittens here. I'm trying to say that your reader has what might be called a cat-awareness of characters and this stretches to about eight or ten characters all told.
More characters than that, and they are going to just slide out of her mind and become, 'Oh. That guy who's going to blow up the hot dog stand.' 'Oh, his old girlfriend.' 'Oh, the bank manager.'
A writer might know and love each of their twenty or thirty people and tell them apart. If the writer was doing a sea thriller he could create two-dozen individual boats and explain this one has this sort of rigging and that one is out of Nantucket. He could give them idiosyncrasies and put memorable names on them.
But the reader, after six or eight cutters and sloops, stops building a picture. Three pages later, she couldn't match the name with the boat. He would have created something the reader doesn't 'see' and wasted his precious words and the reader's focus while doing so.
Characters are a zero sum game. After six or eight, you can add only another character by stealing reality from the ones in place.
One way we end up with excess character-age is we create a separate thread of action that does not have the POV character in it. That action needs a whole new set of characters to run it.
We tell ourselves this is cool stuff. Even though it happens outside the eye range of the POV characters, we must show the reader.
But our MCs can't be there.
This is a job for TELL DON'T SHOW.
Feeding in information the POV character can't see is one of the technique thingums we have to conquer. (And Bloody Hard it is.) We are always trying to get this information to the reader.
But creating scenes to 'explain stuff' is almost always misguided.
The core rule that covers this is that 'scenes arise from the action of our main characters'. Events are important because the MCs react to them.
Think about any of these thrillers with doomsday machines being built. Moonraker, True Lies, X-Men and so on.
Look at a beautifully plot-driven book like any of Agatha Christie's.
(Count her characters, btw, in books that are deliberately plotted with a huge cast. Not twenty-two developed characters milling around. Fewer than a dozen, and those mostly stereotypes. Murder on the Orient Express. Ten Little Indians. A dozen is the outer limit. There is a reason for this.)
Anyhow, see how the action of her books intertwines with the characters.
Doesn't matter how lovely the scene is, how exciting, how important . . . we have to replot to put the POV character there.
Tension doesn't happen because there's a poisonous green glowing vial on an anonymous assembly line in a mysterious factory.
Tension happens because a green glowing vial gets unpacked by the MCs twelve-year-old daughter and set on the kitchen counter.