Numbers 26 - 27

The people were numbered, so that the land could be divided proportionally.  The total census of those able to defend the nation numbered 601,730.  These were all people who were under the age of 20 when the nation decided to NOT enter the Promised Land when God wanted them to do so.  Of that generation, only Joshua and Caleb (and Moses) remained alive.  Joshua was then selected to be Moses' successor.