A question that comes up occasionally in designing massively multiplayer online games is how many characters a player should be able to control on the same game server.
It's an interesting question because it gets into aspects of both business and gameplay design. Fewer characters allowed per server could increase the number of subscriptions purchased, but at the cost of some unhappy players. Some players like to run multiple characters in order to explore different ways of experiencing the gameworld.
On the other hand, if you try to satisfy the Explorers by allowing multiple characters per server, the Achievers will use this feature as well... but they'll use all their other characters (their "alts") to extend the capabilities and possessions of one of their characters (their "main") beyond what that one character alone could normally do or obtain. This reduces the level of challenge intended for the individual player, which is usually balanced for one person playing one character.
So which approach is generally preferable? One character per server? Or many?
I think the answer needs to depend mostly on how easy it is to change an individual character's abilities.
The easier it is for a character to change abilities, the more appropriate it becomes to offer/allow only a single character per server. If today I can be a good crafter, and a few weeks from now I can change into an equally competent fighter, then allowing multiple characters is unnecessary. The bad (unbalancing) effects start to outweigh the positive (exploratory) features.
Alternatively, if character abilities can never be lost, and if characters belonging to the same account can't trade among themselves, then allowing up to three characters (a main and two alts) seems reasonable. The pros and cons balance out.
But some players will insist that three characters are not enough, that six or ten or even more characters per server are necessary to support their preferred playstyle. This might be so; the problem is that what you permit for one player needs to be allowed for every player, and many players will not have any reservations about using multiple characters as robots that subsidize one main character. The more characters per server you allow, the more these utilitarian players will unbalance your game that's balanced around a single character.
Muling is just one aspect of the problem; the complete problem is really that the easier it is for one player to play multiple characters per server, the easier the game becomes.
Allowing many characters, or allowing multiple characters if it's possible to change skills with relative ease, creates the problem of a player being able to play the game as a kind of single ubercharacter. Being able to easily swap between multiple characters with different ability sets is equivalent to having a single character who possesses nearly every ability in the game, and that completely honks up the game's level of challenge which is designed to scale to a single character.
A game that's too easy -- because it's not being played as its designers intended -- becomes boring, and that's one of the many kiss-of-death scenarios possible in these game things. So if anyone asked, I'd counsel a measured approach to the "how many characters per server" question. When forced to choose, I come down on the side of offering as few characters per server as possible.
For a very skill-based game, I'd say one character per server is sufficient. For a very class-based game, I'd suggest allowing up to three, but I'd also make most items no-trade. Furthermore, I'd consider adding code that checks a user's account when trying to log in a character to make sure they don't already have another character active on that server. This will prevent the multiboxers from cheapening their gameplay experience (whether they think that's what would happen or not) by letting it be too easy because they're using their multiple characters as one ubercharacter.
Sometimes what gamers want is not what they should get. And yes, that probably applies to some of the things I've asked for, too.