זֹ֣את בְּרִיתִ֞י אֲשֶׁ֣ר תִּשְׁמְר֗וּ בֵּינִי֙ וּבֵ֣ינֵיכֶ֔ם וּבֵ֥ין זַרְעֲךָ֖ אַחֲרֶ֑יךָ הִמּ֥וֹל לָכֶ֖ם כָּל־זָכָֽר׃
So sollt ihr meinen Bund halten zwischen mir und euch und deinen Nachkommen nach dir: Beschnitten werde bei euch alles, was männlich ist.
Rashi on Genesis
'ביני וביניכם וגו BETWEEN ME AND YOU — “you” is plural and refers to all those belonging to Abraham who were then alive.
Radak on Genesis
זאת בריתי, just as I keep and observe my end as spelled out in verse 7, so you must keep and observe it. The Torah employs the plural mode when speaking of Avraham and his descendants, i.e. תשמרו, as it had done in verse 9 when spelling the word אחריך, "after you,: with the letter י indicating such a plural. This covenant was to be between G'd (sing.) and the Jewish people (ph.)