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.

But David would not number them from twenty years old and under: because the Lord had said that he would multiply Israel like the stars of heaven.

And David hath not taken up their number from a son of twenty years and under, for Jehovah said to multiply Israel as the stars of the heavens.

But David did not take the number of those who were under twenty years old, for the Lord had said that he would make Israel like the stars of heaven in number.

But David was not willing to number them from twenty years old and under. For the Lord had said that he would multiply Israel like the stars of heaven.

David's numbering of the people was therefore a military arrangement in order to fix the amount of his standing army. To the general Oriental prejudice against numbering possessions, etc., there was added in the case of the Jews a special objection - a feeling that it would be irreverent to attempt to count what God had promised should be countless.

But David took not the number of them from twenty years old and under,.... Only those that were twenty years and upwards; but, according to Cornelius Bertram (k), he numbered them that were under twenty, though but sixteen, seventeen, eighteen, or nineteen years of age, provided they were of robust bodies, and of a tall stature, and able to bear arms; which he takes to be the sin of David, in numbering the people, being contrary to the law of God; yet though he had ordered them to be numbered, and they were, yet he would not take them and put them into the account of his chronicles, as in the next verse, that his sin might not be known, see 2-Samuel 24:9.
because the Lord had said, he would increase Israel like to the stars in the heavens; which are not to be numbered, and therefore David sinned in attempting to number the people.
(k) Lucubrat. Franktall, c. 2.

But David took not the number of them from twenty years old and under--The census which David ordered did not extend to all the Israelites; for to contemplate such an enumeration would have been to attempt an impossibility (Genesis 28:14), and besides would have been a daring offense to God. The limitation to a certain age was what had probably quieted David's conscience as to the lawfulness of the measure, while its expediency was strongly pressed upon his mind by the army arrangements he had in view.

1-Chronicles 27:23 and 1-Chronicles 27:24 contain a concluding remark on the catalogue of the twelve detachments into which the men capable of bearing arms in Israel were divided, contained in 1-Chronicles 27:2-15. David had not taken their number from the men of twenty years and under, i.e., he had only caused those to be numbered who were over twenty years old. The word מספּרם points back to למספּרם, 1-Chronicles 27:1. מספּר נשׂא as in Numbers 3:40 = ראשׁ נשׂא, Exodus 30:12; Numbers 1:49, to take up the sum or total. The reason of this is given in the clause, "for Jahve had said (promised) to increase Israel like to the stars of heaven" (Genesis 22:17), which cannot mean: For it was impossible for David to number all, because they were as numerous as the stars of heaven, which of course cannot be numbered (Berth.). The thought is rather that David never intended to number the whole people from the youngest to the eldest, for he did not desire in fidem divinarum promissionum inquirere aut eam labefactare (J. H. Mich.); and he accordingly caused only the men capable of bearing arms to be numbered, in order to organize the military constitution of the kingdom in the manner recorded in 1-Chronicles 27:2-15. But even this numbering which Joab had begun was not completed, because wrath came on Israel because of it, as is narrated in 1 Chron 21. For this reason also the number, i.e., the result of the numbering begun by Joab, but not completed, is not included in the number of the chronicle of King David, i.e., in the official number which was usually inserted in the public annals. בּמספּר neither stands for בּספר (according to 2-Chronicles 20:34), nor does it denote, "in the section which treats of the numberings" (Berth.). היּמים דּברי is a shorter expression for h' דּברי ספר, book of the events of the day.

Because - 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 the truth of his promises.