A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
1 Chronicles 23:1
ch1 23:1DAVID MAKES SOLOMON KING. (Ch1 23:1)
when David was old . . . he made Solomon . . . king--This brief statement, which comprises the substance of 1Ki. 1:32-48, is made here solely to introduce an account of the preparations carried on by David during the latter years of his life for providing a national place of worship.
1 Chronicles 23:2
ch1 23:2NUMBER AND DISTRIBUTION OF THE LEVITES. (Ch1 23:2-6)
he gathered together all the princes of Israel--All important measures relating to the public interest were submitted for consideration to a general assembly of the representatives of the tribes (Ch1 13:1; Ch1 15:25; Ch1 22:17; 1Ch. 26:1-32).
1 Chronicles 23:3
ch1 23:3the Levites were numbered . . . thirty and eight thousand--Four times their number at the early census taken by Moses (see on Num. 4:1-49; Num. 26:1-51). It was, in all likelihood, this vast increase that suggested and rendered expedient that classification, made in the last year of David's reign, which the present and three subsequent chapters describe.
by their polls, man by man--Women and children were not included.
1 Chronicles 23:4
ch1 23:4twenty and four thousand were to set forward the work of the house of the Lord--They were not to preside over all the services of the temple. The Levites were subject to the priests, and they were superior to the Nethinim and other servants, who were not of the race of Levi. But they had certain departments of duty assigned, some of which are here specified.
1 Chronicles 23:5
ch1 23:5praised the Lord with the instruments which I made--David seems to have been an inventor of many of the musical instruments used in the temple (Amo 6:5).
1 Chronicles 23:6
ch1 23:6David divided them into courses among the sons of Levi--These are enumerated according to their fathers' houses, but no more of these are mentioned here than the twenty-four thousand who were engaged in the work connected with the Lord's house. The fathers' houses of those Levites corresponded with the classes into which they [JOSEPHUS, Antiquities] as well as the priests were divided (see on Ch1 24:20-31; Ch1 26:20-28).
1 Chronicles 23:7
ch1 23:7SONS OF GERSHON. (Ch1 23:7-11)
the Gershonites--They had nine fathers' houses, six descended from Laadan, and three from Shimei.
1 Chronicles 23:12
ch1 23:12OF KOHATH. (Ch1 23:12-20)
The sons of Kohath--He was the founder of nine Levitical fathers' houses.
1 Chronicles 23:13
ch1 23:13Aaron was separated--as high priest (see on 1Ch. 25:1-19).
1 Chronicles 23:14
ch1 23:14concerning Moses--His sons were ranked with the Levites generally, but not introduced into the distinctive portion of the descendants of Levi, who were appointed to the special functions of the priesthood.
1 Chronicles 23:21
ch1 23:21OF MERARI. (Ch1 23:21-23)
The sons of Merari--They comprised six fathers' houses. Summing them together, Gershon founded nine fathers houses, Kohath nine, and Merari six: total, twenty-four.
1 Chronicles 23:24
ch1 23:24OFFICE OF THE LEVITES. (Ch1 23:24-32)
These were the sons of Levi . . . that did the work . . . from the age of twenty years and upward--The enumeration of the Levites was made by David (Ch1 23:3) on the same rule as that followed by Moses (Num 4:3), namely, from thirty years. But he saw afterwards that this rule might be beneficially relaxed, and that the enrolment of Levites for their proper duties might be made from twenty years of age. The ark and tabernacle being now stationary at Jerusalem, the labor of the Levites was greatly diminished, as they were no longer obliged to transport its heavy furniture from place to place. The number of thirty-eight thousand Levites, exclusive of priests, was doubtless more than sufficient for the ordinary service of the tabernacle. But this pious king thought that it would contribute to the glory of the Lord to employ as many officers in his divine service as possible. These first rules, however, which David instituted, were temporary, as very different arrangements were made after the ark had been deposited in the tabernacle of Zion.