John Calvin Commentary Genesis 21:31

"Wherefore he called that place Beer-sheba. Because there they sware both of them." — Genesis 21:31 (ASV)

Wherefore he called that place Beer-sheba. Moses has previously called the place by this name, but proleptically. Now, however, he declares when and for what reason the name was given: namely, because there both he and Abimelech had sworn. Therefore, I translate the term ‘the well of swearing.’ Others translate it ‘the well of seven.’

But Moses plainly derives the word from swearing; nor is it of any consequence that the pronunciation slightly varies from grammatical correctness, which in proper names is not strictly observed. In fact, Moses does not restrict the etymology to the well but comprises the whole covenant. I do not, however, deny that Moses might allude to the number seven.