In Andrew Lloyd-Webber's musical, Joseph and His Amazing Technicolor Dreamcoat, one of the first songs introduces Israel this way: Jacob was the father of a whole new nation; because of the number of children he had; he was also known as Israel, but most of the time; his sons and wives only called him Dad!

All kidding aside, Jacob was the father of a nation because he entered into a covenant with God in which that great posterity was part of the promise.  

We have traced the Covenant from the Pre-Earth life, through Adam, Enoch, Noah, Abraham, Isaac, and Jacob. The Covenant is eternal - both new and everlasting. We often speak of the New and Everlasting Covenant as it relates to marriage, but the covenant covers marriage, family, and much, much more.

 Jacob blessed all of his sons before he died.  These blessings are found in Genesis, chapter 49.  Two of the blessing stand out: Judah is promised that kings will be born in his line, including the King of Kings.  

Joseph's blessing was unique in that it was the longest and most complex.  Joseph was told he would be a fruitful bough that would grow beyond the wall. 

The blessings of thy father have prevailed above the blessings of my progenitors unto the utmost bound of the everlasting hill: they shall be on the head of Joseph, and on the crown of the head of him that was separated from his brethren. (Genesis 49: 24) I read this as Joseph receiving the blessings of the fathers, that Abraham sought (Abraham 1: 2)

Jacob also spiritually adopted Joseph's two sons, Ephraim and Manasseh. When he blessed them, he gave the birthright blessing, the blessing of his right hand, to Ephraim, even though Manasseh as older.  When Joseph tried to correct his father's hands, he was told that even though Manasseh would be a mighty people, the descendants of Ephraim would be greater.  Thos descendent play a key role in the winding down scenes of the earth. (Abraham 1:2)

Nephi had the story of Israel on the Brass Plates.  In his writing he adds a clarifying remark that great were the covenants of the Lord which he made unto Joseph (2 Nephi 3: 4)

Nephi also wrote: For the fulness of my intent is that I may persuade men to come unto the God of Abraham, the God of Isaac, and the God of Jacob and be saved. (2 Nephi 3:4)