Have they run out of good angels (Dan. 10:20-21)?
Answer: This cannot be so.
The primary reason this is impossible is that angels are created beings who act in the service of God:
Hebrews 1:14: "Are they [angels] not all ministering spirits, sent out to render service for the sake of those who will inherit salvation?"
If God needed angels to assist this angelic being (whomever he was), all He would have to do is create them. Let us observe what expositor John Gill has written regarding this scenario, with particular emphasis on the identity of Michael:
[There] is none that holdeth with me in these things, but Michael your Prince; Christ the Prince of the kings of the earth, he was the Prince, Protector, and Guardian of the people of the Jews; he is the Angel that went before them in the wilderness, and guarded them in it, and guided them into the land of Canaan; he is the Angel of God's presence, that bore, carried, and saved them all the days of old, and was their King and their God, their Defender and Deliverer, still.
[He] took their part, and was on their side; yea, he was on the side of, and took part with, them that were for them, the holy angels; and there was none but him that exerted his power, and strengthened Gabriel to act for them in "these things" relating to their peace and prosperity: or, "against these" (c), as it may be rendered; against the princes of Persia and Greece, the evil spirits that worked in these kingdoms, in the children of disobedience there; and had it not been for him, and the exertion of his mighty power, it would have been soon all over with the people of the Jews; as it would be now with the church of Christ, of which they were typical, but the Lord is on their side...
Jamieson-Fausset-Brown has this:
To him [Michael] alone of the angels the office of protecting Israel, in concert with the angelic speaker, was delegated; all the world powers were against Israel.
We know that "Michael" could at once summon considerable angelic assistance, just as Christ:
Matthew 26:53: "Or do you think that I cannot appeal to My Father, and He will at once put at My disposal more than twelve legions of angels?"
It, therefore, follows that whatever else the meaning of Daniel 10:20-21, there could never be a shortage of angelic warriors to assist those in need of in their heavenly pursuits.