You can put up to 8 characters on the same server if you get the bigger subscription option (which it defaults to). The only disadvantage being you will get no EXP bonuses on any classes you level on that alt. The game is designed that you can level one character through the game and the progression of all alternate classes will be buffed up by how high your primary class is. With a level 50 character, you can get your alt classes into the 20s in virtually no time at all.

With multiple characters per server, you lose that bonus. To each their own.