Bnei Yisrael as channel of shefa

ויירש זרעך כו' שער אויביו פי' שכל השפע לאומות ע"י ישראל.

"And your offspring shall inherit… the gate of its enemies" (Bereishis 22:17) - meaning that all the abundance (shefa) to the nations comes by way of Yisrael.

The blessing that Avraham's descendants will "inherit the gate of their enemies" teaches that every flow of Divine bounty reaching the nations of the world passes through Bnei Yisrael.

וכל השערים שבא מהם שפע לכל ע' אומות הם נפתחים לבנ"י.

And all the gates through which abundance comes to all seventy nations are opened to Bnei Yisrael.

The "gates" that channel Divine sustenance to the seventy nations are unlocked for, and controlled through, Bnei Yisrael.

וז"ש יצב גבולות עמים למספר בני ישראל כו':

And this is the meaning of, "He set the boundaries of the peoples according to the number of Bnei Yisrael" (Devarim 32:8).

The nations' very boundaries and portions are fixed in relation to Bnei Yisrael, because Yisrael is the conduit through which all the nations receive their share.

Summary: The blessing that Avraham's descendants will inherit "the gate of their enemies" reveals that all Divine abundance reaching the seventy nations flows through Bnei Yisrael, who hold the keys to those gates - which is why the Torah sets the nations' boundaries "according to the number of Bnei Yisrael."