John Gill Commentary Genesis 9:7

"And you, be ye fruitful, and multiply; Bring forth abundantly in the earth, and multiply therein." — Genesis 9:7 (ASV)

And you, be you fruitful and multiply
Instead of taking away the lives of men, the great concern should be to multiply them; and this indeed is one reason of the above law, to prevent the decrease and ruin of mankind; and which was peculiarly needful, when there were so few men in the world as only four, and therefore it is repeated in stronger terms:

bring forth abundantly in the earth, and multiply therein ;
that the whole earth might be overspread with men, and repeopled sufficiently, as it was by the sons of Noah, see (Genesis 9:19) (10:32) .