John Calvin Commentary Genesis 6:19

"And of every living thing of all flesh, two of every sort shalt thou bring into the ark, to keep them alive with thee; they shall be male and female." — Genesis 6:19 (ASV)

And of every living thing of all flesh. “All flesh” is the name he gives to animals of whatever kind they may be. He says they went in two and two. This does not mean that a single pair of each kind was received into the ark (for we will soon see that there were three pairs of the clean kinds, and one additional animal, which Noah afterwards offered in sacrifice). Rather, since mention is made here only of offspring, he does not expressly state the number but simply couples males with females, so that Noah might therefore perceive how the world was to be replenished.