You can transfer up to 8 characters at once from one world to another world. No additional cost for transferring more than one character in single transfer.
Note that you can also give your retainers gil - there are limits to how much each retainer can carry when transferring though.

Note that it can be difficult to transfer gil or items directly from one character to another - you usually need a second account involved to do that. (Either by dual-boxing if that is allowed, or simply by asking a trusted friend to help.)


Balmung is not the only Congested server right now - there are also several Japaneser servers that have been officially declared Congested. Shiva is not one of them, though.

You cannot transfer characters to any Congested server. All other servers are fine for transferring to, even if they are temporarily blocked for character creation.
(Check the Lodestone, or the launcher, for a link to the latest server status information.)