And the sons of Joseph, who were born to him in Egypt, were two. All the persons of the house of Jacob who came into Egypt were seventy.
This little commentary about the number of descendants in Jacob's household is an important reminder about God's promise. God had told Abraham, who was childless at the time, that He would make of him a great nation. And so the promise made then has grown. Form Abraham came one promised heir, Isaac. And Isaac doubled the promise with twin sons, Esau and Jacob. From Jacob came 12 sons and multiple grand kids. By the time they all resettle to Egypt, there are 70 members of the household who directly descend from Abraham. God is keeping His promise a generation at a time.
What I see as the generations spring forth from the Genesis text is a story of the faithfulness of God. When Abraham is first introduced, we have no way of seeing how a nation could come to the old nomad. Yet by bringing one son, the story progresses as God blesses. And the blessing comes despite the character of the people God is blessing. Abraham lies. So does Isaac. Esau has his selfish faults, but we see even more in Jacob. Yet God gives Jacob 12 sons. Those sons become a nation, but not without difficulty, beginning as refugees and then slaves in Egypt. Yet God is faithful to His promise despite human sinful choices and less than ideal circumstances for nation-building.
When God wants to make a nation, He chooses a childless elderly couple to start the process. God uses jealous twin brothers to further the development of His promises. He sends seventy hungry refugees to Egypt to endure generations of slavery. Then He will call up a nation. The means that God uses to fulfill His promise will clearly bring all the praise to Himself. From our point of view, God does it the hard way. From His sovereign purpose, He does it so that He might clearly show His power and be praised.