8 slots with a standard sub, but you can unlock more as the game progresses.
Best thing to do is play through the Tutorial, then head to the Power House (which you get a quest to do).
Once there, you can try out quite a lot of the powers and travel powers, then respec for free. This lasts until you leave the Power House. Once you leave, they become locked, and you have to pay to respec them from now on.