What are the "thirteen covenants made upon" circumcision?

Question

What are the "thirteen covenants made upon" circumcision?

My question came from, "Rabbi Ishmael says: Great is circumcision, since thirteen covenants were made upon it." https://www.sefaria.org/Nedarim.31b.8?lang=he&with=all&lang2=he

Answer 1

In the Torah it says the word bris 13 times (in the chapter commanding it to Abraham)

See halacha 9 in chapter 3 in the Rambam's laws of circumcision

thirteen covenants were established with Abraham, our Patriarch, with regard to circumcision:

1. "I will place My covenant between Me and you" [Genesis 17:2],
2. "And I, behold, My covenant is with you" [ibid.:4],
3. "I will establish My covenant between Me and you" [ibid.:7],
4. "For an eternal covenant" [ibid.],
5. "And you shall observe My covenant" [ibid.:9],
6. "This is My covenant which you shall observe" [ibid.:10],
7. "It will be a sign of the covenant" [ibid.:11],
8. "My covenant will be in your flesh" [ibid.:13],
9. "For an eternal covenant" [ibid.],
10. "He will have nullified My covenant" [ibid.:14],
11. "And I will establish My covenant with Him" [ibid.:19],
12. "For an eternal covenant" [ibid.],
13. "And I will establish My covenant with Isaac" [ibid.:21]

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Talmud Nidarim 31b

Rabbi Yishmael says: So great is the mitzva of circumcision that thirteen covenants were sealed with regard to it, for the word covenant appears thirteen times in the biblical passage that discusses circumcision (Genesis, chapter 17).
(Tosefot and the Ran)