There were fifteen Kings of Arthedain. After the division of Arnor between the three sons of Eärendur, three separate states were created, each headed by a king.

Arthedain was first ruled by Amlaith, the eldest son and heir of the last King of Arnor. Rhudaur and Cardolan were ruled by the other two sons. The kingship passed down from Amlaith, in direct succession, to Arvedui, the last of the Kings.

After the death of Arvedui, there was no longer a kingdom to rule so Aranarth, the king's heir, took instead the title of Chieftain, becoming the first of the Chieftains of the Dúnedain.
Encyclopedia entry originally written by Nienna-of-the-Valar