The text of Deuteronomy 32:8 that the Tsadokite priesthood took with them from Jerusalem to Qumran when they were displaced by the Hasmoneans reads "according to the number of b'nei elohim". This reading is also supported by the Septuagint (the "LXX") Greek translation.
The b'nei elohim were demigods first mentioned in Gen 6:1-4 as the offspring of heavenly beings (angels or gods) with earthly women. In the view of Yair Zakovitch the idea of heavenly beings fathering children with human women was so pervasive in ancient Mediterranean thought that the Bible could not ignore it. He cites the myths of the birth of Hercules, and indicates that there is at least an echo of this myth in the story of the birth of Samson.
As with other myths that the Bible incorporates, the Bible de-fangs the myth, first by denying eternal life to the demigods (Gen 6:3), and then by juxtaposing the story with the wicked generation of the flood (Gen 6:5). The flood apparently puts and end to the phenomenon, but we still see echoes later on, as in the story of the Nephilim seen by the spies sent to scout the Land in Numbers 13:33, and possibly later in the person of Goliath.
Later generations continued to de-fang the myth, for example Psalm 29, where the gods are exhorted to recognize the might of the Lord, and and later on, in Job 1:6, 2:1, where the bnei elohim have been reduced to being obsequious members of His council.
In post-Biblical times a further debunking occurred when the Pharisees interpreted bnei elohim to mean just human charismatic leaders (called judges in Israelite society) based on the linguistic similarity between word in Gen 6:3 translated as "remain" (My life-giving spirit shall not remain in man forever...) and the Hebrew word for "judge".
According to Rachel Elior, even this re-interpretation was not enough for Pharisaic sensibilities, and at some time during the second century BCE, in the course of a general revision of the Biblical texts, one of whose purposes was to expunge the text of Tsadokite elements, the text of Deuteronomy 32:8 was changed to read "b'nei yisrael", the sons of Israel. This change fits well with "Jacob" in the following verse, and with the idea that Jacob went down to Egypt with seventy souls (Deut. 10:22), and that these souls were representatives of all of the seventy nations of humanity, and that the seventy translators of the LXX translation were translating God's word for all of humanity (since everyone who was anyone was assumed to read Greek then).
So it seems that some of the common English translations give precedence to the Qumrani reading over the Masoreti text, at least in this verse. Check the introduction to the translation to see if this is stated explicitly. IMHO it would be only common courtesy for translators to indicate the policy regarding which manuscript they use, and to indicate any deviation from policy in specific verses by footnotes. That would save some confusion and SE.BH questions, though I do enjoy writing answers.
But I still haven't answered your question - Clearly Moses is a dyed -in-the-wool monotheist. Even if we accept the Tsadokite reading of the text, Moses is just recapping history using the common parlance (apparently there was once a myth that humanity was originally divided into nations led by demigods) to say that from the time that humanity was divided into nations (i.e, from the beginning), Jacob was set aside as God's own special portion in humanity.
