John Gill Commentary Genesis 9:1

"And God blessed Noah and his sons, and said unto them, Be fruitful, and multiply, and replenish the earth." — Genesis 9:1 (ASV)

And God blessed Noah and his sons
With temporal blessings, not spiritual ones; for though some of them were blessed with such, yet not all, particularly Ham:

and said unto them, be fruitful and multiply, and replenish the
earth ;
depopulated by the flood: this is a renewal of the blessing on Adam, a power and faculty of propagating his species, which was as necessary now as then, since there were so few of the human race left in the world; and the renewal of this grant was the rather necessary, if, as has been observed, Noah and his sons were restrained from cohabiting with their wives while in the ark: but though these words are not an express command for the propagation of their species, yet more than a bare permission, at least they are a direction and instruction to it, and even carry in them a promise of fruitfulness, that they should multiply and increase, which was very needful at this time.