The limit on the number of virtual users that you can use in your load tests has been removed in Visual Studio 2012. You no longer have to purchase virtual user licenses to increase the number of virtual users that you can use in your load tests. Although you are now unlimited in the number of virtual users you can use, it is recommended that you use a test controller and test agents if you plan to use thousands of users. You will need to determine this for your particular environment.
For Visual Studio 2010 Ultimate, you were limited to 250 virtual users. This limitation could be overcome by either purchasing additional user pack licenses (1000 users per pack), or if with a MSDN subscription by downloading Visual Studio 2012 Feature Pack 2.