“The angel of Yahweh.” See commentary on Genesis 16:7, “angel of Yahweh.”

“many, yes, many.” The Hebrew text has the figure of speech polyptoton, “to increase I will increase.” God was promising Hagar that her descendants would be great in number, and He did that by using the word “increase” (rabah; #07235) twice, in effect, “increase increase.” God had promised Abraham that his seed would be very numerous on a number of different occasions, but this promise is specifically of the offspring that would come through Hagar (Gen. 12:2; 13:16; 15:5; 16:10 (via Hagar); Gen. 17:6; 22:17).

[For more on polyptoton and translating it as many, yes, many, see commentary on Genesis 2:16.]