This is directly from the Sticky on this forum from Yoshi-P: http://forum.square-enix.com/ffxiv/t...s-Restrictions

Each FINAL FANTASY XIV: A Realm Reborn World server has been designed to support a maximum of 5,000 concurrent connections. This is not the maximum number of characters which can be created on a World, however, but the maximum number of characters which can be logged into that World simultaneously. When more than 5,000 characters log into a World at once, there is a higher risk of server crashes and login errors. The restoration of crashed servers is usually followed by a rush of players attempting to log back in, which, in turn, puts a strain on our lobby server, and can ultimately have an adverse affect on all of the Worlds.