As far as we know, there are five for sure. Three normal ones that are always there. One that is just a normal chest with spirit bloom and normal drops at best. And one that only appears if the Templar is not allowed to teleport. I have heard that there is a sixth that you can only get if the sync plates at the very beginning are never taken and no one dies, BUT I have not seen it thus far. And I have to wonder if there is a seventh on the dead gate at the bottom of the jumping puzzle after the gorgon maze. It seems a bit out of the way and you CAN get to it, but there isn't anything there when you get there.