John Gill Commentary Genesis 13:16

"And I will make thy seed as the dust of the earth: So that if a man can number the dust of the earth, then may thy seed also be numbered." — Genesis 13:16 (ASV)

And I will make your seed as the dust of the earth
An hyperbolical expression denoting the great multitude of Abram's posterity, as they were in the days of Solomon, and as they will be in the latter day; and especially as this may respect all the spiritual seed of Abram, Jews and Gentiles, and as they will be in the spiritual reign of Christ, see (Hosea 1:10Hosea 1:11) ; so that if a man can number the dust of the earth, [then] shall your
seed be numbered ;
but as it is impossible to do the one, so the other is not practicable, see (Numbers 23:10) .