So for my story I’m focusing primarily on five characters, all of whom are friends and work together. However I want each character to have his/her own storyline and to grow as an individual. The main characters are a girl and a boy so they get the bulk of the plot. However the third girl is kind of a breakout character and I’ve thought up a plot for her that I’m really excited to write. The last two characters are probably going to be together for most of the story. So that’s four, almost five separate storylines that I want to write. Am I biting off more than I can chew here? Is it possible to make transitioning between the storylines fluid rather than jumpy and tie them all together in the end? Should I maybe just focus on one or two characters and put the other characters in the background and incorporate them as I go?
In the Dragonlance series there are a lot of characters and most have their own stories to tell. Some of the back stories are even made into other books but they also say a lot about each character in the first trilogy. I would say that five isn't too many and if you have well developed characters it could work very well.
You can have as many characters as the story demands. You can have subplots and still not lose focus on the main plot/character. In fact subplots and other chars should ideally enhanced the main plot and the mc(s), else you should have a relook if the chars and the subplots are necessary at all. If they are distractions and not enriching the story, then cut them.
Ask yourself whether these are four stories, or four threads that are all essential to the primary storyline. Every character must contribute materially to the story you are constructing. And take Maia's above advice to heart.
You may have many plotlines you find exciting to write. That's cool. Are they all appropriate to be incorporated in the same novel? Will they all contribute to the main storyline or detract? Will they all interest the reader or distract and slow things down? Do all the characters have to be main characters with their own storyline up front to be told in the same novel as all the other storylines?