Seventy - See Genesis 46:27. The object of the writer in this introductory statement is to give a complete list of the heads of separate families at the time of their settlement in Egypt. See the note at Numbers 26:5.
Matthew Henry
Concise Bible Commentary
During more than 200 years, while Abraham, Isaac, and Jacob lived at liberty, the Hebrews increased slowly; only about seventy persons went down into Egypt. There, in about the same number of years, though under cruel bondage, they became a large nation. This wonderful increase was according to the promise long before made unto the fathers. Though the performance of God's promises is sometimes slow, it is always sure.