As Kysaiana mentioned, you may indeed only use one character: the one you chose during the application.
You may re-copy it, as long as you delete the character first. Peculiarly, at one place, it is stated that one should not delete a character from the test server, but on the other page it says this is what you should do, if you want to re-copy your character over. I'm assuming it is to ask you not to create completely new characters for some reason (which is possible, and helpful even, so I don't quite understand).

Either way, if you can not find this information yourself, here are links to the two most needed pages regarding these questions (both from the TEST SERVER forum):
Best of luck~