Numbering the Children of Levi
Though God had chosen the family of Aaron to serve as the priesthood of Israel, He had also earlier mentioned that the firstborn of all the tribes of Israel were to be His—and therefore directly in His service, evidently to support the work of Aaron's family in caring for the tabernacle and instructing the people in God's ways. But the incident with the golden calf demonstrated the general unworthiness of the people as a whole. Yet at that time, the tribe of Levi, to which Moses and Aaron belonged, stood with Moses "on the Lord's side" (Exodus 32:26 Exodus 32:26Then Moses stood in the gate of the camp, and said, Who is on the LORD's side? let him come to me. And all the sons of Levi gathered themselves together to him.
American King James Version×). And this stand for God was apparently part of the reason that God determined to choose the entire tribe of Levi as His direct servants. They were to, in this sense, replace the firstborn (verses 11-13).
As we have already read, the tribe of Levi was to encamp around the tabernacle, take care of it and administer all the sacrifices and rites (Numbers 1:50-53 Numbers 1:50-53  But you shall appoint the Levites over the tabernacle of testimony, and over all the vessels thereof, and over all things that belong to it: they shall bear the tabernacle, and all the vessels thereof; and they shall minister to it, and shall encamp round about the tabernacle.
 And when the tabernacle sets forward, the Levites shall take it down: and when the tabernacle is to be pitched, the Levites shall set it up: and the stranger that comes near shall be put to death.
 And the children of Israel shall pitch their tents, every man by his own camp, and every man by his own standard, throughout their hosts.
 But the Levites shall pitch round about the tabernacle of testimony, that there be no wrath on the congregation of the children of Israel: and the Levites shall keep the charge of the tabernacle of testimony.
American King James Version×). The Aaronic priesthood was a subset of the general Levitical priesthood—albeit the leading subset. Aaron and his sons were to be the priests, while the rest of the Levites were divided into family groups to serve in the physical areas, such as setting up and taking down the tabernacle and as ushers, porters, teachers, scribes, musicians, officers and judges, etc. (see 1 Chronicles 23).
It is interesting to note in Numbers 3 the different method of numbering used to count the Levites. Whereas the count for the rest of the Israelites was according to men 20 years and up, the Levites were counted even as male infants one month old—and still the Levites were the smallest of all the tribes at only 22,000 men and boys (verse 39). So small, in fact, that there weren't enough Levites to redeem merely the firstborn males a month and older of the other Israelite tribes (verses 40-43). Therefore, the Israelites of other tribes had to make up the difference in money (verse 49).