Lots of Protagonists
The question you should really ask yourself here is whether or not you can handle twelve main characters. Does the idea of it overwhelm you or excite you? If you feel that you’re able to do it, then it’s a challenge I think you should definitely explore. You said they’re very different from each other, but here are some general tips/considerations:
1) Make them distinct in appearance. This doesn’t mean that you need to have twelve different ethnicities represented, but this is your chance to be diverse and embrace backgrounds other than your own. You might also have a good mix of gender or gender identities, as well as different hair colors, eye colors, variations in height and weight, as well as age (in your case, some younger teens versus older teens). Do what you can to make each character unique in appearance, even if that means browsing through Google to find models/actors/drawings for each character (for your own benefit – not to show other people). Even though it’s a novel and readers ultimately won’t see what these characters look like, it’ll be easier for you to describe them each individually if the picture is 100% clear in your own mind.
2) Make them distinct in personality. With twelve characters, it can be easy to fall into tropes/stereotypes. The best tip I’ve heard on this subject is to take two personality traits that don’t seem to go together and put them in one person. Someone can be friendly, yet selfish. A character who’s really tough might still be dependent on other people. The “funny” one can actually be intelligent and thoughtful, as opposed to just the dumb friend. Challenge yourself to come up with traits that make it easier to distinguish them, and if you do have any similar traits, try to put those in characters that aren’t easy to get mixed up in appearance.
I also gave another tip way back when someone asked about archetypes, as a good starting point to develop your characters. See this link I referenced to read more about those. The key is starting with an archetype and then adding/changing aspects of each one to make it unique.
3) Give them very different names. As a general rule, I try not to have two many characters whose names even start with the same letter, but at the very least, try to avoid names that appear or sound too similar, like a character named Sammy and another named Sandy. Or one named Christian and another named Christine. Names are an easy way to show distinction, so don’t pass it up.
4) Consider their backstories, but don’t go crazy. Show differences in the way they were raised, their hobbies, the friends they had, whether or not they had siblings, and whether they went through any trauma or significant hardship. I warn you not to go crazy, because of twelve people, not all of them are going to have a tragic backstory, so make sure that some of them experienced normalcy and happiness (perhaps up until your story begins).
5) Introduce them slowly. Usually, the wisest advice in such large casts of characters is to start with the characters before they all know each other. So you can focus on building their personal story/character development one at a time before their individual stories converge into the main plot. If your story is set up so that they all know each other from the start, you can still introduce them in batches. Start your story with a few of the characters together in one scene, and really spend time showcasing each character in that scene. Once you’ve well into the scene, you might also mention a couple other characters, who you’ll actually bring into the story a scene or two later. Just try to avoid putting all twelve of them into the very first chapter, and if at all possible, try to keep all twelve from being key players in one scene. The important thing is allowing a reader time to get to know each other character on their own or in small groups, instead of trying to differentiate them when they’re all in one scene.
6) Don’t get too attached. Be prepared to cut some of your characters, if their significance to the plot later on seems small or nonexistent. If each character is important and has a contribution to the plot overall, then that’s great. But if some of them just kind of fade into the background and do very little to advance the plot, you might have to consider cutting the character out. Of course, that doesn’t mean you can’t ever use the character. If your hesitation to cut a character is because you love the character, consider putting them in a different story where they’re able to snag more of the spotlight.
All in all, I think you should give it a shot. The number of main characters in a story is largely dependent on how many characters the author is able to handle, because it’s that which will determine how successful the story is written. If you’re not overwhelmed by the idea, you should do it, but be prepared that as the story progresses, you might find some characters aren’t as necessary as they first seemed.