Adam Clarke on Genesis 24:60

Be thou the mother of thousands of millions - לאלפי רבבה lealphey rebabah, for thousands ten thousand, or for myriads of thousands, a large family being ever considered, in ancient times, as a proof of the peculiar blessing and favor of God. Similar addresses to a daughter, when she is going from her father's house to live with her husband, are very common among the Hindoos; such as, "Be thou the mother of a son," "Be thou the wife of a king," etc. See Ward.