A sept is a subdivision of the Aiel clans. Aiel can be of the same clan, but different septs. Each clan has different septs. For example, Jindo is a sept of the Taardad and no other clan would have a Jindo sept. While these are the known septs, it is possible that there are many that are known only within each clan.
While it is not known how many septs a clan normally has, the Shaido has had as many as eighty-three septs WH 4.