The next two chapters of Numbers focus on counting Levites, two different censuses (censi?) for two different purposes. But it opens with an account of the sons of Aaron, or more broadly, the line of priests. Although Aaron is from the tribe of Kohath, a Levite tribe, the priests should be thought of differently than the Levites. Ultimately, the priests are responsible for the mediation between YHWH and His people, only they may handle the holy items and enter the the holy place. The Levites, as we shall see detailed, are not permitted to do those things, but are responsible for the guarding and transport of the tabernacle and its contents.
We are reminded that Aaron’s sons Nadab and Abihu died interacting with “strange fire”, basically by offering a sacrifice that YHWH did not ask for. It sets a tone for the next few chapters which show dire circumstances for folks trying to interact with people or things that are set apart for YHWH’s service in ways that He hasn’t permitted. All told, Aaron has only two sons left, thus Eleazer and Ithamar will serve as priests under Aaron.
YHWH then speaks to Moses and tells him to charge the tribe of Levi with serving the priests, specifically by protecting the priests/contents of the tabernacle as well as to move it when the tribe moves (don’t forget the context, Numbers is the preparation for God’s people to move into the land He has promised). As part of this protection, anyone who comes near to what the Levites have been tasked to protect shall be put to death. The consequences of allowing the tabernacle, its implements, or its minsters to be made unclean can have consequences that extend to the entire camp and must be dealt with quickly.
In v.11, we get an explanation of why the first census of the Levites is occurring (which obviously must be different than the census that has already occurred for the non-Levites, which was to get counts for battle). We are reminded that, ever since the 10th plague in Egypt, the first born of everything belongs to YHWH, including the first born of Israel, who were to be dedicated to His service in the sanctuary. But ever since the issue with the golden calf in Exodus 32, the Levites as a tribe were identified to take the place of the first born of everyone.
So, now we get the count of each of the tribes of the Levites (sons of Levi), Gershon, Kohath and Merari. There is a hierarchy to the order. The priests will face east (most favored), the Kohathites south (next), then the Gershonites (west), and finally the Merarites to the north. They also have different responsibilities over the tabernacle items when they travel. Kohathites carry the holy items like the ark of the covenant, the altar, the tables, etc., the Gershonites the curtains, the Merarites the poles and tent pegs. When they marched, you’d normally expect the more favored roles to go first, but in order that the tabernacle be reconstructed, the Merarites go first to get the pegs and poles up, then the Gershonites put the curtains on, then the Kohathites deliver the holy items so the priests can get all of that set up.
After all of the counts, there were 22,000 Levites and 22,273 of the rest of Israel. This means there’s a deficit (if the Levites are supposed to stand in place for all the first born of Israel, they are 273 fellas short).
These remaining lives must be redeemed (since they haven’t been substituted) and the cost of redemption is five shekels per head (or what was the going price for a servant at the time). The money went to Aaron and his sons.