I am not sure, as the phrasing leaves it open ended, IMO. My first thought was that the Sea Folk got 20 teachers in perpetuity, but they are supposed to stay for a year each and not allowed to leave unless another comes to replace her. The books do not make it clear if that goes for only an early departure during her year, or if she needs to be replaced at the end of the year. I don't have my copy of KoD handy, but check if Zaida tells Logain how long Amylia will be there. Does she say for a year OR she gets a replacement, or after a year AND a replacement comes.