The cast of characters should be as large as it needs to be to tell the story. Full stop.
As far as making each character unique is concerned, your characters don’t have to be special snowflakes, as exclusive of one another’s personalities and interests as the Bratz dolls. You don’t need to separate your characters into neat tropes like The One Who Likes Sports and The One Who’s Super Into Science, etc, though that is a perfectly valid way to do things. Your characters’ differences can be more subtle.
For example, let’s look at eight Harry Potter characters: Harry Potter, Ron Weasley, Hermione Granger, Neville Longbottom, Luna Lovegood, Ginny Weasley, Draco Malfoy, and Cho Chang. These are all pretty distinct characters in the Harry Potter series, but they each have things in common (or not) with the others which drives plot.
Draco is the foil of Harry, but both struggle under weight of society’s expectations.
Harry, Ron, Hermione, Neville, and Ginny were all sorted into Gryffindor, an indication that they are all brave and strong-willed (though it takes Neville a hot minute to work up to the strong-willed bit).
Cho Chang and Luna Lovegood are both Ravenclaws, so they both value wisdom. However, Luna is a loner, while Cho is popular.
While we’re on the subject of popularity, Draco and Ginny are both popular as well.
Both Ginny and Cho are described as beautiful. Both love to play Quidditch. Both date Harry at some point.
Harry and Neville were both born at the end of July. A prophecy made by Trelawney could have applied to either of them. They were both raised by someone other than their parents.
Harry and Ron both hate doing homework, while Hermione loves it. All three end up working for the Department of Magical Law Enforcement.
Luna and Draco are both blond. Ron and Ginny, siblings, are both redheads.
The list can go on and on. I guess my point (other than the fact that I love Harry Potter) is that each character can have a lot stuff, personality or otherwise, in common with the others while remaining unique. You would never mistake Luna for Cho, Hermione for Ginny, or Harry for Draco. They’re so different, and not because they fit neatly into boxes which cut away any connection they might have to each other, but because what they have in common affects each of their characters differently.
The other thing that each of these characters has in common is that they are important to the plot. If you take any one of them away, the plot is diminished. If you can take any of your characters away and still have a complete plot, do it. Simplify. If not, keep it as is.
I don’t know if this answers your question, but I hope it helps!
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