There were actually thirteen tribes because the tribe of Joseph was split into two, namely Manasseh and Ephraim (Genesis 41:5–52; Genesis 48:1–22). The tribe of Manasseh was then also further divided into two, with one half of the tribe living west of the Jordan and the other half east of the Jordan. (The Levites did not receive a portion of the land).
2 Their inheritance was by lot, just as the LORD had commanded by the hand of Moses for the nine and one-half tribes.