We know there are only five council members, and that they change from time to time (i.e. Terion => Mellar => ??). Of the known council members, only one that we've met is still alive (The Demon). What is known of the others is now on the Jhereg Council main page, since the Council Members are described and named in Dzur. It could be that the Number One Jhereg Councilmember is also accorded the rank and title of being the Jhereg Heir, but given how Jhereg aristocratic rank is portrayed as being utterly distinct from rank within the Jhereg Organization, this is very, very tentative.