I'll try to explain this but I'll probably bungle it up or overcomplicate it somehow. Also this is just based off of information gleamed through information available to the public such as interviews and presentations. As the game is still in beta it is all still subject to change.
At launch they'll be a total of 32 different souls split up among the 4 callings. So 8 Rogues, 8 Mages, 8 Clerics, 8 Warriors and you'll be to unlock all 8 for your calling eventually.. When you first create your character however you'll only be able to choose 1 of the 3 basic souls available for each calling. In the beginning you choose what you were in your past life from one neutral soul, one Defiant Faction soul, one Guardian Faction soul.
For example Jim decides to start as a Defiant Warrior. The 3 basic start souls for Warrior are Champion, Reaver, and Paladin. But since he's from the Defiant Faction he can only choose from Champion and Reaver when he first creates his character so he decides to start as a Reaver. At level 6 he can unlock the Champion soul and becomes a Reaver/Champion. As Jim plays he can unlock the other 6 warrior souls including Paladin which was the Guardian Faction's starter soul. At any point in time he can have a load out of 3 active souls. Jim can choose to swap and respec those 8 souls to his heart content. So he can swap from a main tank Paladin/Void Knight/Reaver combo to a Riftblade/Paragon/Champion DPS machine.
Nothing to see here people, let's keep the line moving. >.>