According to the number of the sons of ...

In Haazinu:

ח בְּהַנְחֵל עֶלְיוֹן גּוֹיִם, {ס} בְּהַפְרִידוֹ בְּנֵי אָדָם; {ר} יַצֵּב גְּבֻלֹת עַמִּים, {ס} לְמִסְפַּר בְּנֵי יִשְׂרָאֵל. {ר}

8 When the Most High gave to the nations their inheritance, when He separated the children of men, He set the borders of the peoples according to the number of the children of Israel.

But why should the nations be distributed, and have borders, in accordance with the number of the children of Israel? And there are famously 70 nations. Where do we find 70 Israelites? Only when they went down to Egypt. Mah Inyan Shemitta Etzel Har Sinai?!

Ibn Ezra writes that it was 70, which divided in the generation of the Dor Hapalagah. And seventy would be sufficient, the same as the number of the children of Israel. Thus:

[לב, ח]

בהנחל עליון גוים -

אמרו המפרשים:

על דור הפלגה שנפצה כל הארץ כי אז גזר השם להיות ארץ שבעה גוים לישראל, והוא שתספיק למספרם על כן למספר בני ישראל

He continues, a bit later, saying that there is something very deep here:

וטעם למספר בני ישראל –

כאשר אמרו חז"ל:

כי צורת יעקב אבינו חקוקה בכסא הכבוד.

והוא סוד גדול והעד כי חלק ה' עמו וזו היא המעלה הגדולה שלא עשה כן לכל גוי, על כן אמר: הלה' תגמלו זאת עם נבל ולא חכם שנפל ממעלתו באולתו, וזה טעם אביך קנך כי קנהו לכבודו וילמדהו, כאשר יפרש כמעשה האב עם הבן.

At the moment, I don't understand the deep idea Ibn Ezra is hinting at. Perhaps I will check supercommentaries later. But not just Ibn Ezra is cryptic. The Biblical text is strange as well -- why should they be divided lemispar benei yisrael?

Rashi also understands this as 70 being in accord with the number of the children of Israel, but that Hashem spared them in the Dor Haflaga because of the eventual few, the nation of Israel, which would come about. Thus:

When the Most High gave nations their lot: When the Holy One, Blessed is He, gave those who provoked Him to anger their portion, He flooded them and drowned them [i.e., that was their lot].		בהנחל עליון גוים: כשהנחיל הקדוש ברוך הוא למכעיסיו את חלק נחלתן הציפן ושטפם:
when He separated the sons of man: When [God] scattered the Generation of the Dispersion [which built the tower of Babel], He had the power to remove them from the world [altogether], but He did not do so. Rather, “He set up the boundaries of peoples,” [i.e.,] He let them remain in existence and did not destroy them.		בהפרידו בני אדם: כשהפיץ דור הפלגה היה בידו להעבירם מן העולם ולא עשה כן, אלא יצב גבולות עמים קיימם ולא אבדם:
according to the number of the children of Israel: [God let man remain in existence] for the sake of a [small] number of the children of Israel who were destined to descend from the children of Shem, and [the sake of] the number of the seventy souls of the children of Israel who went down to Egypt, He “set up the boundaries of peoples,” [i.e., He separated man into seventy nations with] seventy languages.		למספר בני ישראל: בשביל מספר בני ישראל שעתידים לצאת מבני שם, ולמספר שבעים נפש של בני ישראל שירדו למצרים, הציב גבולות עמים שבעים לשון:

It works out as peshat, yet the idea that it refers to a holding back of punishment within a specific otherwise unnamed incident does strike me as somewhat midrashic.

Shadal, as a pashtan, initially has difficulty accepting this, but eventually comes around to it. Thus:

ח ] יצב גבולות עמים למספר בני ישראל : ימים רבים היה רחוק בעיני לקבל פירוש רש"י ורז"ל ( עי ' ילקוט נח רמז ס " א ) שהציב שבעים אומות כמספר שבעים נפש שירדו למצרים , ועכשו רואה אנכי כי כל שאר הפירושים הם נגד מליצת הכתוב . והפשט אינו אלא כפירוש רש " י . והנה ידוע כי כיון המספרים בענינים רחוקים זה מזה היה דבר חשוב הרבה אצל כל הקדמונים , כגון רמ " ח מצות עשה כנגד רמ " ח איברים ( מכות כ " ג ע " ב ) וזולתם רבים , והנה קודם שיזכיר מה שעשה ה ' לטובת ישראל , ימצאוהו בארץ מדבר וכו ' וכו ', הקדים ואמר כי ה ' חלק האומות לשבעים דוקא כנגד שביעם בני יעקב , וזה סימן שישראל חשובים ככל העולם כולו , כי חלק ה ' עמו גו '

a
He manages to come to terms with the seeming randomness of it by pointing out that there are these special numbers, such as ramach eivarim. Thus, 70 just like the 70 who descended to Egypt.

There is a tantalizing possibility which presents itself in the Septuagint, as a textual variant. They translate the pasuk as follows:

8. ὅτε διεμέριζεν ὁ ὕψιστος ἔθνη ὡς διέσπειρεν υἱοὺς Aδαμ ἔστησεν ὅρια ἐθνῶν κατὰ ἀριθμὸν ἀγγέλων θεοῦ

Instead of "sons of Israel", they have "angels of God." What textual variant could spark this? The answer is benei Elohim instead of bnei yisroel. And then, it works perfectly with the idea of angels, sarim, being appointed for each people.

Compelling of course doesn't mean true, and indeed sometimes is a reason to suspect a variant as being contrived to solve a problem...

{
Update: As YAW reminds me in the comment section, not only does the LXX have this, indicating a variant, but in the Dead Sea Scrolls, we can actually see this variant, with lemispar benei Elohim. As noted earlier, this works out with the idea of sarim, guardian angels in charge of different nations. Still, even without the Dead Sea Scrolls, we could guess from the LXX that such a variant exists. While a tantalizing possibility, I don't see it as absolute that the real answer is that the Masoretic text is incorrect and that we should read like the Dead Sea Squirrels and the LXX.
}

I also saw a creative idea in Chizkuni, that lemispar benei yisrael does not refer to the 70 descendants who went down to Egypt, but to the twelve sons of Yaakov who inherited the land eventually. Of course, there are more than 12 nations, but there are twelve sons of Canaan.

After all, we see in I Divrei Hayamim 1:13-16:

יג וּכְנַעַן, יָלַד אֶת-צִידוֹן בְּכֹרוֹ--וְאֶת-חֵת.	13 And Canaan begot Zidon his first-born, and Heth;
יד וְאֶת-הַיְבוּסִי, וְאֶת-הָאֱמֹרִי, וְאֵת, הַגִּרְגָּשִׁי.	14 and the Jebusite, and the Amorite, and the Girgashite;
טו וְאֶת-הַחִוִּי וְאֶת-הַעַרְקִי, וְאֶת-הַסִּינִי.	15 and the Hivite, and the Arkite, and the Sinite;
טז וְאֶת-הָאַרְוָדִי וְאֶת-הַצְּמָרִי, וְאֶת-הַחֲמָתִי. {ס}	16 and the Arvadite, and the Zemarite, and the Hamathite. {S}

These are eleven. And then from one family, the Perizi, who are not mentioned here, split off. If so, despite the twelve shevatim not being present then, the Canaanites were. And the sons of Canaan were servants to the sons of Shem, and so the Canaanites could not complain, because mah shekana eved kana rabbo. All in all, a cute and neat explanation of how "in accordance to the number of the Bnei Yisrael" works out.

Rabbi Yonatan Eibeshitz, in Tiferes Yehonasan, has a fun and (I think) creative explanation somewhat related to l'mispar b'nei yisrael. I don't think it is correct, but it is still interesting. Some of the text is hard to read, so what follows is my rough summary and reconstruction.

He explains that it is known to astronomers {/astrologers?} that in each Mazal {zodiac sign?} has six big stars. Since there are twelve Mazalot, 12 X 6 = 72. Except, at the time of the Mabul {and this is a famous Midrash}, Hashem took two stars from the Kima constellation and thus, 72 - 2 = 70. And so too, the Israelites went down with 70 souls to Egypt, for Hashem said to Avraham, "so {many as the stars} shall be your descendants." Later, that promise encompassed the greater number of all stars, while initially it was only parallel to those 70 big stars.

And thus, when the pasuk says בְּהַנְחֵל עֶלְיוֹן גּוֹיִם, {ס} בְּהַפְרִידוֹ בְּנֵי אָדָם, this is the generation of the Flood, as it says in the Targum. For this comes from the Mabul, in which 2 stars were taken from Kima. But in truth, there needs to be 72, corresponding to 72 names. And how so? The 70 nations, plus also Israel is split into two nations. Yisrael, which is the portion of Hashem, and Yaakov who follow after Yisrael, tied to them like with a rope {chevel}. And this is what is meant by the next pasuk,

ט כִּי חֵלֶק ה, עַמּוֹ: {ס} יַעֲקֹב, חֶבֶל נַחֲלָתוֹ. {ר}

9 For the portion of the LORD is His people, Jacob the lot of His inheritance.

Where כִּי חֵלֶק ה עַמּוֹ refers to Yisrael, and יַעֲקֹב חֶבֶל נַחֲלָתוֹ is Yaakov.