With NaNoWriMo here, I'm trying to actually write that novel that's been bubbling in my head for the last six years. The problem (two separate related problems, actually) is that I don't know where to introduce characters. The first of the problem is that I don't know when to introduce the main character's mentor/father figure. If I introduce him too late, then he's only there for the big battle that ends the 'first book' and is the main character crossing the First Threshold. If I introduce him too early, then it's almost too coincidental. He's already only joining because he thinks the girl the main character is helping reminds him of the daughter he killed in a fit of Herculean rage brought on by his cursed sword. The second of the problems is when to bring in the villains, or at least how to have them show up and get in the hero's way without feeling like they're dominoes, and that they come when the one before them dies. I want them to feel like they work for the same group, but at the same time I don't want them to come at the hero en mass. Related is the fact that I don't know how many characters I should have. There are seven villains, and likely at least 5 heroes.
First, you need to decide why it's important that the father is in the story. Introduce him at a point when it seems only natural to do so. Second, as far as your character count goes I would recommend focusing on one MC and one or two villains. The others can stick around but maybe don't give them so much limelight. It gets confusing when you're reading something with so many characters you can't keep track of who's who.
He's there because they needed a mentor, and eventually he'll serve as a counterpoint when the main character meets his real father, and learns that after his mother left the man, he stopped being the war hero he was when she was with him, and became a drunk. The villains numbers aren't so difficult. Most of them aren't even a threat to the characters, and at least two of them don't really care. I need enough heroes to counter them, though, because the Big Bad's whole thing is that he's going to destroy the world (he'll make it better afterwards) unless it rises up against him. The main character inadvertantly picking up a multiethnic crew of heroes symbolically shows that the world is willing to work together, despite what his life as a soldier taught him. Then again, sometimes it's more like I'm writing an RPG than a novel. Which may be the story's final fate, depending on things.
It sounds like this is a novel that needs to be written from a few POVs. Two main characters and the main villian perhaps. By showing us the main villian, you can show us why he is sending the bad guys out one at a time so that we don't feel like they are dominos. Give the mentor a reason to wait. Maybe he gets captured. Maybe the heros save him as they are doing something else. They go to steal a magic sword, and see the mentor in a dongeon for example.
