Ex 1:1 Now these are the names of the children of Israel who came to Egypt; each man and his household came with Jacob:

 2 Reuben, Simeon, Levi, and Judah;

 3 Issachar, Zebulun, and Benjamin;

 4 Dan, Naphtali, Gad, and Asher.

 5 All those who were descendants of Jacob were seventy persons (for Joseph was in Egypt already).

 6 And Joseph died, all his brothers, and all that generation.

 7 But the children of Israel were fruitful and increased abundantly, multiplied and grew exceedingly mighty; and the land was filled with them.

The point here is to show the growth of the children of Israel numerically. Initially, they were a small group in Egypt but they grew prolifically and rapidly into a significant number in Egypt as the years rolled by. From "seventy persons" to being "fruitful and increased abundantly, multiplied and grew exceedingly mighty; and the land was filled with them."

This is in fulfillment of the promises of God to Abraham, Isaac and Jacob, that they would have numerous descendants.