The meaning is, David, when he desired to number the people, he designed to number only those who were from twenty years old and upward, or (which is the same thing) those that drew sword, 1 Chronicles 21:5, and not those who were from twenty years old and under. He would increase Israel like to the stars of the heavens; and therefore to number them all both above and under twenty years old, had been both an infinite trouble, and a tempting of God, or a questioning of the truth of his promises. And possibly this circumstance might in part deceive or quiet David's conscience, that his desire of knowing the number of his people did not proceed from distrust of God's promise or providence, but from a prudent care to know the true state and strength of his kingdom.
Related Commentaries of 1 Chronicles 27:23
1 Chronicles 27:23
23 But David took not the number of them from twenty years old and under: because the LORD had said he would increase Israel like to the stars of the heavens.