[Quick Summary: A 12 yr. old Kahki Scout and his girlfriend run away together, in hot pursuit by parents, and a scout troop.]
I'm not a big fan of Wes Anderson films, but I do like this script.
It abides by (then stretches) my two rules of ensembles:
1) Even in an ensemble, there are 1-2 lead characters. *
Here, Sam and Suzy Bishop (the kids) are clearly the leads.
They easily could have been overshadowed by four very active adult characters, but the script is careful to keep the focus on the children.
2) Multiple subplots are fine, but they must SUPPORT the main character's story.
Scout Master Ward, Captain Sharp, Mr. and Mrs. Bishop each have their subplots. That's right - there are FOUR additional arcs.
However, the reader never forgets about Sam and Suzy.
How did the writers do it?
The supporting characters have separate subplot arcs, but each one supports the main plot.
ex. Scout Master Ward loses his status --> rescuing the kids restores his honor.
ex. Captain Sharp loses the woman he loves --> rescuing the kids gives his life new meaning.
ex. Mr. and Mrs. Bishop have a troubled marriage --> rescuing the kids brings them together.
WHAT I'VE LEARNED: Strong ensembles have strong (but not overpowering) subplots.
Moonrise Kingdom (2012)
(The above link has an interactive script, plus a separate PDF.)
by Wes Anderson and Roman Coppola
*If you insist on multiple main characters, please read my post on The Best Years of Our Lives (three main characters face the same problem).
Those types of stories need much more structure to keep the story clear and unified.