Complete number and completely serve the Lord

1 Chronicles 27:1-34
Heads of families, commanders of thousands and commanders of hundreds, and their officers, who served the king in all that concerned the army divisions that were on duty month by month throughout the year. Each division consisted of 24,000 men.
Benaiah is son of Jehoiad the priest and the commander for the third month who was a mighty man among the Thirty and was over the Thirty.
All 12 army commanders seem to have come from David's special guard of 'mighty men'
The officers in charge of the tribes, those in charge of the king's property - storehouses, produce and livestock - and his personal advisers.
Why?
Those under 20 were not eligible for military service and so were never counted.
'King's friend' is an official title.
12 tribes, 12 months, 24000 men for each division, 12 Levites in 24 divisions of Priests,12 members in  each 24 singer group. Twelve is a complete number in Bible.
How?
Manage a nation need many capable men, same as church need many honest Christians to serve God and honor God's name.