To address your very last sentence Alex, yes, there is only one angel of the Lord, period. Secondly, "The" angel of the Lord "NEVER" appears in the New Testament as "the angel of the Lord."
You referenced Matthew 1:18-20. Notice Matthew 1:24 and how it's worded, "And Joseph arose from his sleep and did as "the" angel of the Lord commanded him." Even though the word "the" is used this is still not "the" angel of the Lord because vs20 clearly states "an" angel of the Lord appeared to him in a dream."
The same can be said regarding Stephen at Acts 7 where he mentions the angel of the Lord by quoting the Old Testament. But first notice Acts 7:2, "And he said "hear me, brethren and fathers! "The God of glory APPEARED to our father Abraham when he was in Mesopotamia, before he lived in Haran."
Stephen is giving the Jews a history lesson. Now notice Vs30, "And after forty years had passed, "AN" angel appeared to him/Moses in the wilderness of Mount Sinai, in the flame of a burning bush." At vs31, "there came the voice of the Lord." Vs32, "I am the God of your fathers, the God of Abraham, and Isaac and Jacob."
At vs33, the Lord ask Moses to take off his sandals. Skipping down to vs37. "This is the Moses who said to the sons of Israel, God shall raise up for you a prophet like me from your brethren. Now for vs38, "This is the one who was in the congregation in the wilderness together with "THE" angel who was speaking to him on Mount Sinai. In essence, the Jews were rejecting God Himself.
What is also noteworthy is what Stephen says while the Jews were stoning him at vs59, "And they went on stoning Stephen as he called upon the Lord and said, "Lord Jesus receive my spirit."
At this point I think it's important to define the difference between the little word "an/a" and the word, "the." The chief grammatical function of "an" (or a) is to connote a thing "NOT" previously noted or recognized. "The" connotes a thing "PREVIOUSLY noted or recognized.
The first appearance of the angel of the Lord as "the" angel of the Lord is at Genesis 16:7. He multiplies Hagar's descendants at vs10. Hagar says she saw God and lived to tell about it at vs13.
At Genesis 17:1-2 the Lord God Almighty physically appears to Abraham and God multiplies his descendants. This multiplying Abraham's descendants is reinforced at Genesis 22:17. If you read at Genesis 22 starting at vs11-18 you will discover that it is the angel of the Lord who multiplied Abraham's descendants and swore an oath to Abraham (vs16) to seal the deal, so to speak.
The writer of the book of Hebrews at Hebrews 6:13-17 references this swearing from Genesis 22 and is clearly identified as God. Hebrews 6:13, "For when God made the promise to Abraham, since He could not swear by no one greater, HE SWORE BY HIMSELF, vs14, saying, I will surely bless you and I will surely multiply you."
If the angel of the Lord is just that, an actual angel he could not have sworn the oath to Abraham, angels cannot swear oaths on behalf of God Almighty Himself.
The following site shows many of the early Church Fathers believed the angel of the Lord was the preincarnate Jesus Christ. http://catholicnick.blogspot.com/2019/05/is-angel-of-lord-in-old-testament.html Now, this is a Roman Catholic site and after reading it I realized I posted my comments on this site back in 2022. You can find them at the very bottom when I'm the last poster. Btw, I am "NOT" a Roman Catholic, far from it.