This is really one of those controversial issues where there's no point in us taking sides. There are lots of reasons why one character per server is good and also bad. We think the answer is to have this functionality part of the ruleset associated with the server, not the game.
Some team members prefer to play in a single character per server world for a variety of reasons (most especially so that you could have different factions/races/nationalities without players easily being able to play different sides, although a second account still gets around this).
But for others, that’s not true – some are willing to give up the flexibility of trying out all sorts of character combinations because they tend to fixate on one character. Others here at Sigil, however, LOVE to have multiple characters in the same world, to play one of them one day, then dabble with another combination the next. If you told them they had to do that on different worlds for each character, well, they'd not respond kindly.
So there's really no 'right' or 'wrong'... this is a preference, and preferences can generally be accommodated by varying the non-core rules on different servers.
Source: Official FAQ
There will be max per shard, but we're not releasing that yet. We are also toying with the idea that once a player has a character(s) above a certain level on a shard, additional slots open. This also is related to possible RP and other alternate rule set servers which only allow one per shard, race war servers that only allow a subset of races to be made per shard, etc. Lots of cool ideas, IMHO.
Source: Aradune Mithara 10-05