Some Dungeon Masters may have a preference to this, what's yours? When it gets to be too many people it must be hard to control regardless to live or Play By Post. Probably in LARP it doesn't matter although you might need an equal number of the enemy.
To me it does not matter really you having a nice mix of characters is a good way to make a game go smoother. I have done role play with one character all the way up to 14 characters at a time. I can tell you that he more characters and the more people it does get interesting at times.
6-8 is probably best, and I'm more thinking about 8 that way if someone has to leave for an emergency there still will b a significant number of players to carry on the campaign.
In my opinion the best number is a couple more than you planned because if anyone was to leave, as has already been suggested, then you will still have a full compliment.
In a Play By Post environment you get people coming and going all the time because the commitment isn't there like it is if you were in person so you should have 25% more players than you really want to have.
I've experienced in my current game a player leaving and a player joining while adventuring. For sure you should always have more than less because some players have unexpected life commitments. I agree with Hunter, if this were live role-playing people will be embarrassed to go and come without any commitment.
Edited: Txtrpg on 20th Jul, 2015 - 12:57am
My rule is more based on player experience and the challenge level of the session.
The more serious the challenge level, the more experienced players are needed.
The more light hearted and easy going the session, the less you need experienced players.
The average number of players I suggest is 6 to 8. Which allows for a serious 4 player session and an easy going 12 player session.
Good discussion. I would be curious to know how many active players a Dungeon Master/GM has been able to maintain through a campaign.