1)
Why does it say "l'Mishmaros"?
1.
Rashi: We find (Divrei ha'Yamim I, Perek 24 and onwards) that the Leviyim were divided in 24 Mishmaros.
2)
What is the significance of the number of Leviyim listed here?
1.
Malbim (8): Only four of these signed on the Chasum (10:10-14). There were 17 Leviyim signed on the Chasum; perhaps the seven Leviyim in 11:15-20 completed 24, to head the Mishmaros. Or, the five added names here (starting with Yehudah) complete 22, to match the number of Kohanim; "Echav" (8) includes Vakvukyah's brothers Cheresh and Galal mentioned in Divrei ha'Yamim I, 9:15, so there were 24 in all.
