Here we hear again the number 7. We first find it in the week of creation. The rest of the Bible indicates that this indeed is a special number: of fullness. Nothing needs to be added to the arrangement of the world nor to this divine protection. This is enough.
15 Then the LORD said to him, “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And the LORD put a mark on Cain, lest any who found him should attack him.