Jacob's 70
When Jacob went down to Egypt he was 70 in number. How many were part of Israel when they came out of Egypt? The 70th book, Numbers, tells us.
Exodus 1:1-7
Count
70 = Numbers
Numbers
When Jacob went into Egypt he was 70 in number. When he came out of Egypt he was estimated to be about 2.5 million in number. That's a huge increase. We know the rough number because the book of Numbers records a census count of those who left Egypt.
The total from the census count, the 603,550, is only counting males old enough to serve in the army (meaning those in their 20th year and up) and excludes the Levites who were not to serve in the army. If you add the Levites and a wife and two kids for everyone, we arrive at around 2.5 million people. That's a big increase from the 70 that went to Egypt with Jacob. The book of Exodus is not kidding when it says "The sons of Israel were fruitful and increased abundantly, and multiplied and grew exceedingly strong; and the land was filled with them."