2 Chronicles 31:17

And the enrollment of the priests [was] according to the house of their fathers, and the Levites, from twenty years old and upward, according to their posts by their working groups.

1 Chronicles 23:24

These [were] the sons of Levi according to the house of their fathers, the heads of the {families}, according to their enrollment, by the number of the names, according to their head count, [who were to] do the work of the service of the house of Yahweh, from twenty years old and above.

1 Chronicles 23:27

For by the last words of David, they [are] the number of the sons of Levi, from twenty years old and above.

Numbers 3:15

"Muster the descendants of Levi according to {their families}, according to their clans. You will count every male from {one month} and above."

Numbers 3:20

The sons of Merari according to their tribes: Mahli and Mushi. These [are] the clans of the Levites according to {their families}.

Numbers 4:3

from {thirty years old} and above, up to {fifty years old}, everyone who comes to the service to do the work in the tent of assembly.

Numbers 4:38

And the descendants of Gershon counted according to their clans and according to {their families},

Numbers 4:42

Those counted of the clans of the descendants of Merari according to their clans, according to {their families},

Numbers 4:46

All those counted of the Levites whom Moses and Aaron and all the leaders of Israel mustered according to their clans, according to {their families},

Numbers 8:24

"This [is] what is for the Levites: {those twenty-five years old} and above will come to help [with the] service in the work of the tent of assembly;

Numbers 17:2-3

"Speak to the {Israelites}, and take from among them twelve staffs, {a staff from each family} from among all their leaders according to their families' households. Write the name of each man on his staff,

1 Chronicles 24:20-26

And as for the remainder of the sons of Levi: of the sons of Amram, Shubael; of the sons of Shubael, Jehdeiah.

2 Chronicles 31:2

And Hezekiah appointed working groups of the priests and the Levites, each according to their divisions, {according to the service for priests and Levites}, for burnt offerings, for peace offerings, to serve, to give thanks, and to praise in the gates of the camps of Yahweh.

Ezra 2:59

Now these were the ones who came up from Tel-melah, Tel-harsha, Cherub, Addan, and Immer, but they were not able to prove their fathers' household and their descent, if they were from Israel:

Treasury of Scripture Knowledge

Summary

Both to the genealogy of the priests by the house of their fathers, and the Levites from twenty years old and upward, in their charges by their courses;

Bible References

Genealogy

Numbers 3:15
"Muster the descendants of Levi according to {their families}, according to their clans. You will count every male from {one month} and above."
Numbers 4:38
And the descendants of Gershon counted according to their clans and according to {their families},
Numbers 17:2
"Speak to the {Israelites}, and take from among them twelve staffs, {a staff from each family} from among all their leaders according to their families' households. Write the name of each man on his staff,
Ezra 2:59
Now these were the ones who came up from Tel-melah, Tel-harsha, Cherub, Addan, and Immer, but they were not able to prove their fathers' household and their descent, if they were from Israel:

Twenty

Numbers 4:3
from {thirty years old} and above, up to {fifty years old}, everyone who comes to the service to do the work in the tent of assembly.
Numbers 8:24
"This [is] what is for the Levites: {those twenty-five years old} and above will come to help [with the] service in the work of the tent of assembly;
1 Chronicles 23:24
These [were] the sons of Levi according to the house of their fathers, the heads of the {families}, according to their enrollment, by the number of the names, according to their head count, [who were to] do the work of the service of the house of Yahweh, from twenty years old and above.

By their courses

2 Chronicles 31:2
And Hezekiah appointed working groups of the priests and the Levites, each according to their divisions, {according to the service for priests and Levites}, for burnt offerings, for peace offerings, to serve, to give thanks, and to praise in the gates of the camps of Yahweh.
1 Chronicles 24:20
And as for the remainder of the sons of Levi: of the sons of Amram, Shubael; of the sons of Shubael, Jehdeiah.