Genesis 16:10
And the angel of the LORD said unto her, I will multiply thy seed exceedingly, that it shall not be numbered for multitude. And the angel of the LORD said unto her, I will multiply thy seed exceedingly, that it shall not be numbered for multitude.
The angel's promise to Hagar came after she was directed to go back to Sarah and submit herself to her. It came after she was assured that the Lord had heard and regarded her affliction. In that context she was told that her seed would be multiplied exceedingly, until it could not be numbered for multitude. That assurance of innumerable descendants was given to comfort her in her distress.