Sons of Israel or sons of God
Whether the Most High set the nations' bounds by the number of Israel's sons or of the sons of God.
Deuteronomy 32:8-9
8When the Most High gave to the nations their inheritance, when he separated the children of men, he set the bounds of the peoples according to the number of the children of Israel.
9For Yahweh’s portion is his people. Jacob is the lot of his inheritance.
What each tradition says
Rabbinic / Midrashic
Rashi explains that “according to the number of the children of Israel” means because of the future children of Israel descending from Shem’s sons. In accordance with the seventy souls of Israel who went down to Egypt, He firmly established the nations’ boundaries as seventy separate nations.
Rashi (Solomon ben Isaac), Rashi on Deuteronomy - Pentateuch with Rashi's Commentary (M. Rosenbaum & A. M. Silbermann trans., 1929-34; Sefaria) — Comment on Deuteronomy xxxii. 8, third clause
Patristic
Clement of Rome states that God fixed the bounds of the nations according to the number of the angels of God. He adds that His people Jacob became the portion of the Lord, and Israel the lot of His inheritance.
Clement of Rome, The First Epistle of Clement to the Corinthians (ANF vol. 1) — Ch. XXIX (the quotation of Deut. xxxii. 8-9)
Catholic
Haydock says God suffered the people of Canaan to occupy as much land as would be requisite for the Israelites. He notes that the Septuagint reads “according to the number of the angels of God.”
George Leo Haydock, George Haydock's Catholic Bible Commentary (1859) — Comment on Deuteronomy xxxii. 8
Orthodox
Origen says other nations are called a part of the angels. He cites that when the Most High divided the nations and dispersed the sons of Adam, He fixed the boundaries of the nations according to the number of the angels of God.
Origen of Alexandria, De Principiis (ANF vol. 4) — Book I, Ch. 5 (On Rational Natures) - the nations and the angels
Reformed / Protestant
Gill takes the clause to mean that a land was measured and bounded sufficient for the twelve tribes of Israel when numerous and due to inhabit it. Meanwhile it was given to Canaan and his eleven sons to possess, not as their inheritance, but as tenants at will, until the proper heirs existed in sufficient number and age to inherit.
John Gill, John Gill's Exposition of the Whole Bible — Exposition of Deuteronomy xxxii. 8, on the clause "according to the number of the children of Israel"
Evangelical
Jamieson, Fausset & Brown note that although Peleg is not named, a division of the earth is noticed. The relation to the number of the children of Israel may point to the 72 (12 x 6) names that occur in the text.
Jamieson, Fausset & Brown, Commentary Critical and Explanatory on the Whole Bible — Note on Deuteronomy xxxii. 8-9, second part (on the number of the children of Israel)