'Eight' in the Bible

Every male among you who is eight days old shall be circumcised throughout your generations, [including] a servant whether born in the house or one who is purchased with [your] money from any foreigner, who is not of your descendants.

So Abraham circumcised his son Isaac when he was eight days old, just as God had commanded him.

Bethuel became the father of Rebekah. These eight [children] Milcah bore to Nahor, Abraham’s brother.