The players are the game

Characters. You need some. But how many? And who? To decide what characters your story needs, keep in mind that your characters are your plot. They make it happen. Imagine a football game. Your main character is the star player. He's on the field (the setting) holding the ball (the action). The field posts are his goal. His motivation is to score and achieve his goal. The main character carries the action toward the goal. Naturally, it wouldn't be much of a game if he were alone on the field. Let's give him a team-mate to help him. (In your book, this will be a side-kick or romantic interest). This team-mate should complement the main character; he should have strengths and weaknesses that are different, to keep things interesting. But don't look now! Here comes the rival team's best player! (This is your villain in a suspense or mystery novel or the competition in a romance novel.) Be sure that the rival has as strong desire to keep your hero from achieving his goal as your hero does to reach it. These characters are all you need. Any other characters should be added only if they serve to advance the plot. Remember, a close game is more nail-biting than a rout. Keep your sides evenly matched. Victory is sweeter when snatched from defeat. Consider this: the player has no function outside of the game. The game cannot exist without the players. Your characters and your plot are inseparable in the finished book, even if we tend to work on them one at a time in the planning stages.