Why this is a mistake: If the curtain on a play opened and there were thirty people on stage and all of them had speaking roles, would you be able to identify and keep track of everyone? Or if you went to a party and opened the door and the room was filled with people you’d never met before, would you have a good time? Throwing too many characters at the reader creates the same sense of bewilderment and diminishes the reader’s ability to empathize with any of them. Larry McMurtry can do it in Lonesome Dove and win the Pulitzer Prize, but we’re not Larry McMurtry. He’s able to do it because he makes each character distinctive. It’s a question of how much you’re able to change personalities with your characters. Most of us can only take on a handful.
The solution: Before you start writing, decide how many characters you feel you can handle in the story. You will have your protagonist and your antagonist. Then you will have your named supporting characters. Named characters will be those who appear throughout the story.
It’s probably not a good idea to give names to characters who appear only once. Those characters might be described as spear carriers, analogous to those people on stage in the opera who stand in the background, carrying—you got it—spears. They’re window dressing, and you can describe them by their roles, such as “the taxi driver” or “the desk clerk” so as not to confuse the reader.
Make sure the reader can keep track of your named characters, and keep focus on the protagonist and antagonist.