Make sure you really need as many main characters as you are proposing. If a character doesn't demand front and center time, that means they have little to contribute (therefore they aren't "main"). If you just have a lot of side characters, that's not quite the same problem see the three posts authmentors posts for that discussion Handling a Cast of Thousands (1 of 3). Main characters are your marquee personalities, the high powered heavy lifters with some measure of significance to the story and a detectable amount of plot armor. Also, keep in your mind the distinction between protagonist and main character. The protagonist is a viewpoint character usually charged with being a part of, if not actually solving, the plot problem.
So, a main personality contributes to resolving one of the plot threads, and will be in focus doing the needful things. If the story is military drama and taking the hill is critical, it's not necessarily the guy that charges the machine gun nest that gets the focus. He's the red-shirt that takes the bullet for the hero that ULTIMATELY takes the hill. We can see the action through the hero who has bonds with the hapless red-shirt and feel torn up and angry when he dies. It's the death that fires up the hero to do that critical needful thing. Because of these elements that's why the hero IS IN FOCUS.
So the reader can experience these critical nuances. Note here, the red shirt will get a some scene focus. He'll show us the photo of the girl friend, talk about opening up a grocery store back home... all these bit-character-who-is-about-to-die tropes. As experienced story consumers we all say to ourselves, 'here it comes, someone is about to croak.' We spend time on the character so the reader and hero experience some connection and attachment. This way when he charges the machine gun nest and dies, it fires up the hero. I want to emphasize here he didn't get screen time because of the ACT of bravery, or the death, BUT BECAUSE HIS ACTIONS MOTIVATE AND CHANGE A PROTAGONIST CHARACTER.
THAT is how characters get in focus. They contribute to pushing the plot forward. Make sure your primary cast members do things that affect the story outcome.