Forbidden Siren handled 10 main characters really well. Granted Only 3 of them were consistently playable but it still did it well in my opinion.
While I personally wish Mio had been the third main character in 3 because of it being her story to tell and development to receive (As it is the Lost Village section is more of a recap that doesn't go anywhere in the games' storyline rather than the character development and extra information like the Himuro Mansion section is for Miku), I think they tied Kei into the story well enough for it to be work.
His camera abilities aren't that great, and his specific ability is the most tacked on thing ever, but he does make the story go along and helps tie Mio and 2 to 3's storyline (He should have still been in the game but instead they should have had him take Mio to see Rei to help both deal with sudden loss which would have allowed her to be involved in the main plotline like it would make sense thematically for her to be) and he's a very likable character so it isn't so bad.
I think Fatal Frame 4 did a great job with 4 main characters. Yes Madoka only got one chapter playable and died afterwards, but they still manage to make her a very deep and tragic character with all of the optional story elements you can find out about her in chapters 2, 3, 4, and 5. It makes the complete ending with her much more heartwarming and effective. They dropped the ball on some character interaction moments but otherwise I feel it did it really well because everyone was important to the main storyline.
I think if they can just take their time and really think through what they're writing they can make a perfected multi character storyline. It's more exciting for me to see all these different characters and finding out how they intertwine with the main narrative so if they actually have them all be important to the main narrative then I want them to stay.