I have six flat-characters and two-rounded, the protaganist and his love interest. Eight characters is pushing the boundaries, when five or six is considered to be the norm. But they are all vital to the plot and storyline.
A brief explanation of what I mean to novice writers: A flat character is a minor character in a work of fiction who does not undergo substantial change or growth in the course of a story. Also referred to as "two-dimensional characters" or "static characters," flat characters play a supporting role to the main character..
A round character is a major character in a work of fiction who encounters conflict and is changed by it. Round characters are more fully developed and better described than flat, or static, characters.
I must admit I find it easier to brand characters by whether they have a POV ie. you get to know what they're thinking, for however brief a time, though you have to be careful with this.
In one of Tom Connery's 'Markham of the Marines' novels he gave two Spanish infantrymen thoughts and the characters only appeared in a single paragraph, which really grated on me. So MCs have a POV, secondary characters may or may not, depending on their importance to the story, and tertiary characters none at all.
I like David's 'character profiles' but I'm far too lazy to spend the time. Perhaps they're more important if you write as 'omniscient narrator' who by definition knows all and thus must aware of the finest detail of character lives at all times. Thinking about it, maybe I write single person POV out of laziness ;)
Tradtional classical novels, and best selling contemporary novels have one main character, whose name is usually in the title. I am sure there are many you could list. These best selling novels may have a strong cast of characters, but the storyline and plot are about an individual hero or heroine.
Many consider flat-characters to be round when they are not. Flat characters have a vital role to play, but they should not be fleshed-out, or steal the limelight from the main character. To do so would undermine the main character. For example, my evil king commits many heinous crimes and nearly kills my protaganist on three separate occasions, but he is a flat character.
In general, most novels concentrate on one person, not two, three or four. You cannot have more than one main character of equal importance, otherwise, who or what is the novel about? Who is the glue that holds the novel together? One individual has to stand head and shoulders above the rest. He or she is central to the novel. They are like a magnet that attracts the iron-filings, or like the axle of a wheel with all the spokes pointing towards them.
Victoria, You are correct, it as about the function of characters, the roles they play, and what defines them from each other. Also, in a series of novels where the timeline covers many generations, I would expect there to be many protaganists.
Jonathon, I had to many characters and diverted from the plot and storyline. Reducing the number of characters kept my pliot and storyline straight, and as simple as possible.
David, you can flesh-out round characters as long as they do not takeover the novel from the protaganist.
Oh yes - and...
My "personnel files" record for me not just the conflicts and solutions that go on in the story for people but conflicts and solutions (or lack of them) that they have had in the past. I am so glad that I can do all this digitally! Even there it can be a nightmare at times. (If I change something significant about a character I have to complete all the cross-references in the files as well as in the plots... Which seriously encourages me to not do it!)
At one stage, a couple of years ago, I became completely stuck in the main plotof the enormous story. Eventually I realised that I had a whole pile of off-shoot stories for almost everyone as well as the main characters.
The thing is that with a post-apocolyptic scenario everyone will have got to where the story is by their own route. To go back over-and-over the sequence of events would tend (strongly) to get monotonous - although it has been done - at least in film. The solution I came up with is to create a Video-room for each character to tell their story to camera.
This may or may not work as stories but it certainly helped me...
This raises the question of how much writing we need (or don't need) to do around the edges - or just as simple practice...
David