Well, I would first reconsider why your story needs that many prominent characters. Stories with a big cast can work, but you may find that investing development in only a few characters (instead of stretching your scenes and jumping around from character to character) would be better.
Secondly, I'd like to hear your definition of a "main character". Main characters are called as such because they are the centerfold of the story - everything is told from their perspective or they have significant purpose in the plot. Without them, there really is no story to be told. Being a secondary character, however, doesn't necessarily mean the character has no purpose or is less "important". Secondary characters can be crucial to the main character's development or be vital to the story, while still remaining in the background. Just because they have the label of being the "back up" cast doesn't mean they're expendable or don't deserve their own story.
If you find that you really do need all of these "main" characters, then I'd first start prioritizing which characters are going to require the most on-screen development time, or maybe even which characters you find yourself writing about the most. Sometimes when you write, even if you don't think you'll have favorites, you'll enjoy writing about certain characters more than others. Those will be the ones that appear often and if you're not careful, can overshadow the other supposed "main" characters.
Properly portraying them all at once is a tricky thing though - you can either break their points of view up chapter by chapter, or you can use the classic scene breaks when changing perspectives. Just don't try to go over the same plot event five times because you want to get every character's impressions of the scene. That's really tedious for the reader to have to blaze through just to move on with the story, and you'll bore them to tears regardless of how interesting you think each character is.
Background story is all up to you. Some characters may need more back story than others; you'll just have to find out by experimenting with their characters.