Now that you've identified most of your characters, the major ones and the secondary ones you need to decide who is telling the story. You can choose one character, two or many but when in their head or heads, you need to make them clear to the reader. Otherwise you could need a cast of characters to refer to when reading the book.
How are you going to do this? Show them in a scene. Establish their name right away. This will help the reader focus. Also give them traits, ways of speaking or other things to help clearly identify the viewpoint character in the reader's head.
Josie paced the room. She spoke aloud and took another slug from her glass. "How could he do this to me?" She slammed the glass on the table and ran her hands through her short curls. Hopefully the familiar gesture would help her find an answer.
Stay in a character's view point for a scene, a few paragraphs. Don't bounce from one person's viewpoint to another's in the space of a paragraph. You don't want to give your reader whiplash.
My own choice is to use one character per scene, even when I have more than two or three viewpoints in a book. I've sometimes used as many as twelve viewpoint characters in a book but when I'm in their head, what I must do is view everything through their eyes. What they see, hear, taste, touch or smell belongs to them not to the other characters.