There are supposed to be 9 classes on Beta and 12 upon release with the possibility or more in the future. Below is a video taken from the Russian Beta release of the 9 open classes in the training system.
Keep note though peeps, it's important to level everything as that gives your character more stats and perks, those carry over no matter what class. Plus the way the system is setup is you start with only Paladin, Lightbender and Cryomancer unlocked. Once you start playing you gather gems and points to allow you to unlock the other classes which vary from basic, intermediate or advanced. The higher the "tier" the more it will take to unlock but they claim it shouldn't take more than a few days of short play to unlock each at a time. Edit: The three missing classes are supposedly going to be Lancer, Witch and Alchemist.
