John Gill Commentary 1 Chronicles 23

So when David was old and full of days
Perhaps was now in the last year of his age, about seventy years old, though before he was bedridden; see (1 Chronicles 28:2) ,

he made Solomon his son king over Israel ;
declared him to be his successor; this was before the affair of Adonijah, for then he ordered him to be anointed king, and placed on the throne; and this aggravated the rebellion of Adonijah, that it was against the declared and known will of his father.

And he gathered together all the princes of Israel
Ordered them to be assembled, partly to acquaint them with his will and pleasure, that Solomon should succeed him in the kingdom; and partly to settle the order of the work and service of the temple; and therefore these were convened

with the priests and Levites ;
to assign to them their respective places and offices.

Now the Levites were numbered from the age of thirty years, and
upward
So they were numbered in the days of Moses, and by his order, and so they were now at first; though afterwards there was a new reckoning of them, (1 Chronicles 23:24)

and their number by their posts ;
or, according to their heads:

man by man, was thirty and eight thousand ;
which vastly exceeds the number in the times of Moses; see (Numbers 4:47Numbers 4:48) , but then those above fifty years of age were not numbered, as they now might; since their work in the temple would be much more easy, no burdens to carry, as the ark, the vessels

Of which twenty and four thousand were to set forward the work
of the house of the Lord
To prepare for the work of the priests by slaying the sacrifices, flaying them, cutting them in pieces, andwashing them, and bringing them to the altar; 1000 of these served weekly in their turns.

and six thousand were officers and judges :
that acted as justices of the peace in the several parts of the country, heard causes and administeredjustice to the people, being trained up in and acquainted with the laws of God, civil as well asecclesiastic; some were more properly judges, and others executioners of their sentence; see (Deuteronomy 16:18) (17:9).

Moreover four thousand were porters
At the east, north, and south gates of the temple, in their turns;

and four thousand praised the Lord with the instruments ;
were singers in the temple, which in all made up 38,000:

which I made, said David, to praise therewith ;
which instruments he devised and ordered to be made to praise the Lord with; see (2 Chronicles 29:26) (Nehemiah 12:36) .