Sometimes, it's hard to figure who exactly your main character actually is.
Janet Fitch, my professor at USC and author of White Oleander
, tells a great story about writing that book. The main character was not originally Astrid, a young girl whose life is continually upended by her crazy, absentee mother who murders a man with poison. Instead, the story began with the mother, but Fitch realized that the mother didn't have a story - she was a murderer, sure, but she was unrepentant about it and to change that would alter the character. The mother was this great, complex character, but there was no plot to her, no place for her to go. As such, Fitch created Astrid, to struggle against the mother and gave her a growing up arc.
The point of Fitch's story is this: as writers, we often do not know who the main character is when we start writing. A lot of times, it changes as we go along, or another character comes up and demands attention while we are writing. They are just so interesting that they take over the stage, and our planned out main character fades into the background.
However, it is important to note that you've got to know for yourself who the main character is at some point (hopefully before a first draft is finished). While interesting characters are great, characterization is only one part of a book. Characters work hand-in-hand with the plot to create a narrative, and without a main character directing the action of the plot, your narrative falls to pieces.
The point I actually took from Fitch's story is that you must find a narrative that works. By taking the main character and altering it, you need to also alter the plot. By choosing a main character, by saying this is the person the reader needs to pay attention to, then you've already dictated at least part of your plot. The main character must change between page 1 and the end. If not, then you have no story. Or you have the wrong main character.
This isn't to say that you can't have multiple main characters. The problem is still the same - they must evolve through the plot. If they are a main character, we need to see every step of their narrative arc. Just remember, though: the more main characters you have, the more in-depth and creative you're going to have to be with plotting, as the reader will expect there to be a reason that you have multiple main characters. If you don't know what that reason is, I suggest you find one, hopefully through the plot and not just the theme.