Albert Barnes Commentary 1 Chronicles 27:23

"But David took not the number of them from twenty years old and under, because Jehovah had said he would increase Israel like to the stars of heaven." — 1 Chronicles 27:23 (ASV)

David’s numbering of the people was therefore a military arrangement to determine the size of his standing army. In addition to a widespread ancient aversion to numbering possessions, the Jews had a special objection—a feeling that it would be irreverent to attempt to count what God had promised would be countless.