In the Sword of Truth novel series Kahlan is introduced as the Mother Confessor. Is she the Mother Confessor because she is the last one alive? Is she the Mother Confessor because she is the most powerful? Or does she hold this title for some other reason?
Kahlan Amnell is, at the time of the Sword of Truth novels, the last Confessor alive. When there were other Confessors, the Mother Confessor was often chosen because her power was the greatest. Kahlan was chosen, by the Council of the Midlands to be the Mother Confessor because her mother's gift of Confession (who was also once, the Mother Confessor) was also strong in her.
Since the power of the Confessor was focused on the revelation of the truth, and came with the terrible price exacted on people exposed to their power, most people feared and respected the decisions of the Central Council of the Midlands. To their credit, the Council of the Midlands did everything they could to ensure the Confessors were women of that were well educated, trained for their careers and aware of the cultures and traditions of the Midlands.
It was known that anyone exposed to the power of the Confessors became their obedient servant and would do anything she asked, up to and including taking their own life in any manner the Confessor asked. This power could not be resisted and could not be rescinded. If a Confessor was forced to use her power it was rare that she could use it more than once or twice before becoming exhausted, so when hunting parties were used to destroy the Confessors, they were sent out in parties of four members, because most confessors could only affect one person, perhaps two before succumbing to exhaustion.
It is not known if Kahlan Amnell was the last Confessor but it is rumored that a later book will reveal descendants of Kahlan will inherit the power from her.