Since my last post I think 6-10 may be best. It may make tracking players more difficult for the Dungeon Master but it makes Play By Post run smoother for players since some of those will not be as frequent at least there might be a core 3-4 that keeps a steady flow of roleplay happening.
The "Sweet spot" in terms of players for me, whether that be Play-by-Post, live games, or games played online seems to be around three or four. Having too many beyond that seems to make it more difficult to really incorporate the players/characters and make them relevant.
With too many players, the risk is too great that I will overlook the actions or dialogue of some them; though truth be told I can end up doing this sometimes even with only three players. A lot time and effort as a GM seems to go into keeping track of player actions, how NPCs and the world around them will react to them, and maintaining some sense of consistency in the world in relation to character actions.
When there are too many players coming and going it becomes easy to forget some of the details regarding actions that have been taken and how those actions might have impacted the overall game. This can lead to some inconsistencies later on in the game if/when I forget for example that so-and-so's character said something that flattered and NPC and then later that same NPC doesn't seem to remember the interaction, or it just doesn't come up.
I tend to believe that players generally want the actions of their characters to be overall memorable and to have some sort of lasting impact on the game; not every action mind you, but most 'scenes' serve a purpose and the characters job is to have an impact on that scene and GM needs to make sure to note that impact and apply later at an appropriate time if necessary.
The more players, and therefore characters, are involved the more effort the GM has to put in to keep up and maintain the consistency and cohesion of the game. Of course it depends on the game that one is playing as well. A simple hack and slash can support far more players than a game that relies heavily on narration and character development; which is the style I tend to prefer and that is probably why my 'sweet spot' is around three to four players.
I can definitely see the need to determine the number of players you want to have and then adding a few to that number when advertising though because of issues related to player retention. If I want three players I should probably advertise for five or six because you always do seem to have those that are sort of 'fly-by' players, particularly in play by post, that don't last longer than a few actions before they disappear. But even face to face and online games I've been involved in have suffered from losing players after only a few sessions.
I think three or four players works well in "Face to face". My group is down to two players, and it has suffered from that. Too often one is unable to attend, so there's no point continueing the game and we play something else. When we were six it was problematic for a different reason - people chatting instead of following play.
We did have a new player for a few sessions, but he dropped out. I was talking to him recently and it turned out that he had thought he was supposed to be "Acting" - doing everything in dialogue, as the character. I assured him he can just say "My character tries to… ", the In Character: stuff is chrome, and we may get our third again…
Sad thing: Our old third, the first person I introduced to Role-playing Game's in this village, has been lost to World of Warcraft. That someone prefers to sit at a computer rather than run or play a game with their friends… tragic.
Depends on if you want quantity or quality in your Role-playing Game. You can have loads of players just doing nothing and just a few making everything come alive.
Fairmaiden is right. I am in a two player game with Icon and we are moving fairly fast. In the game with more players that had many come and go we are not moving as speedily.
Bindoner has a good point about players misinterpreting what's expected of them but we're aren't mind readers so you have to ask questions. Just going silent and disappearing isn't the solution. Seems like when you ask a question on this site most everyone is willing to help.
Aericsteele made some good points. From what I've seen the players that tend to stay are the ones who have been here longer or done more. If they just joined the site it seems like they're more likely to disappear.
I think in this format six to eight is the best number. As others have said that number will soon fall anyways. Many more than that and it is hard to keep it together unless it is mainly combat with little role playing. In a perfect world I feel that four to six commited players who post frequently and regularly would be ideal. That would be enough for a good mix of characters but not too many to bog things down.