To organize your essay, I would start off by introducing the novel in general, giving a one or two sentence summary, and then at the end of that first paragraph, a thesis that answers the question. So, "There are at least five symbolic mockingbirds in the novel," could be your thesis sentence. Then, in each succeeding paragraph, talk about one or two of those mockingbirds, and why they are considered to be so. Then, after you have discussed all five, add a concluding paragraph where you summarize things.
It also helps to understand the mockingbird symbolism before you get started. Atticus at one point in the novel says, "It’s a sin to kill a mockingbird”, and B.B. Underwood, later,
“likened Tom’s death to the senseless slaughter of songbirds by hunters and children."
These two quotes indicate that a mockingbird is a beautiful and innocent creature that is cruelly harmed because of society's pride and fear.
The most obvious mockingbirds in this novel are Arthur "Boo" Radley, and Tom Robinson. They were both innocent people were were unfairly "shot down" or mistreated by their society. It was a sin to judge them and harm them so, just because of fear and pride. But, the people did it anyway. The other less obvious candidates for mockingbirds could be Tom's wife Helen, as she is harassed by Bob Ewell after the trial, and has to unfairly alter her life because of his cruelty. Another is Mrs. Dubose, who is unfairly judged by Jem and Scout, when at the heart of her struggle is an attempt to overcome addiction. Another possibility is in Mayella Ewell, who, even though she made the wrong choices in the end, was unfairly and cruelly beat by her father, just because she was living her life and was very lonely. Those are five potential candidates for your essay. I have also provided a link below that gives more detailed instructions on how to write in-class essays, and that should be helpful too. Good luck!