In this chapter, the fighting age men are counted by tribe and the Levites are assigned their duty as the custodians of the tabernacle.
There's so much stuff in this chapter I want to talk about, I hardly know where to start. I guess I'll start by talking about the census from a high level and then discussing the details of how it was implemented and some thoughts about the results. I will conclude by discussing the assignment of the Levites, which is significant in itself.
One minor note before I begin is that this chapter occurs "on the first of the second month, in the second year", while later in Num 9, the people celebrate the Passover on the fourteenth of the first month, which means that Numbers is not entirely chronological. The general organization of Numbers seems to be mostly chronological with occasion interruptions as the author wishes to demonstrate some point or other.
Moving on, why does God command Moses to take a census of the Israelites? Fundamentally this seems to be a military action, as evidenced by verse 2: "Take a census... every male, head by head from twenty years old and upward, whoever is able to go out to war in Israel...." This is not a census of the people, this is a census of soldiers. Remember that Moses has already instituted judges and leaders over thousands, hundreds, fifties and tens (Ex 18:21). This is both a judicial system (as originally instituted) but also a military organization, and as we can see from the census, the military organization roughly falls along tribal lines (not shown here, but probably also subdivided by clan and family as implied by v. 2).
What is the purpose of the census though? I would guess that the reason for a military census is to help with the allocation of forces during the invasion of Canaan. That is, if you know how many soldiers are in each tribe/clan/family, then you can assign blocks of troops to invade this or that town based on the estimated strength of resistance in the town. Without knowing how many soldiers are in each tribe, planning such an invasion would be much more challenging. So to the best of my knowledge, that's why this census is happening.
To repeat what I stated in the introduction, this chapter begins Numbers on a very militaristic tone. While Exodus was really more a book of departure, this book initiates most of the major plans for attacking Canaan. We will see this planning continue later as the people get closer to the promised land itself.
Next, I think it's noteworthy that the LORD assigns a tribal representative from each of the twelve tribes. This is an important administrative step because of the latent conflict between the twelve tribes, which I occasionally wrote about in my analysis of Exodus. Most of the laws in Exodus try to emphasize the unity and equality of the Israelite tribes which are inherently fractious. I like to think of the tribes of Israel as being the collective embodiment of their namesakes, the twelve sons of Israel. And they threw Joseph into a pit because they were jealous of him, and Joseph held Simeon hostage in Egypt. And Reuben slept with his father's concubine as a power play, which would have been primarily aimed at his rivals (brothers). Remember, the father's inheritance is divided amongst the sons, so your brother's wealth is your own poverty. So yeah, there were rivalries amongst the founders and now there are rivalries amongst the tribes.
The LORD stipulates tribal representatives, then, to allow all of the tribes to oversee the census and ensure that it is taken fairly, without overestimating oneself or underestimating one's rivals. I can imagine that the tribes would have been skeptical of a biased process unless they "have their man" on the leadership team. That's possibly another reason why Levi is excluded, because Moses and Aaron are Levites so they would be suspect if their own tribe were included in the census. From a more practical perspective, a tribal leader will be more capable of counting his own tribe rather than assigning someone to count a tribe he is not within, so this makes logistical sense as well.
One other thing I'd like to point out is that since these representatives are "the leaders of their fathers' tribes" and "the heads of thousands", they are clearly tribal elders and probably old and respected men.
Now I will discuss the actual results of the census. The results of the census are perhaps more interesting than they would appear at first glance for two reasons. First, it gives us a sense of the overall population of the nation as it travels to the promised land. Since we are told the population of men in a certain age range, it is possible to roughly extrapolate the total population (including women and children) using statistical models. Most estimates of this sort place the Israelite nation at around two million people, which is really pretty impressive. It's as if some major city like Chicago were to collectively pack their bags and walk a few hundred miles to Ohio, except instead of marching through midwestern prairies, they are marching through Middle Eastern deserts. Having lived for several generations in the farmlands of Egypt, they are clearly unprepared for the challenges of the journey, and this shows through their many complaints about lacking water and food. With two million people in an ecosystem that is not designed to support them, this is really a very perilous journey, which exaggerates their dependence on the manna and water from God even more.
The second result is that we can see the relative strength of the different tribes. This helps foreshadow the future significance of each of the tribes, with a few exceptions. Note that Judah, for instance, is by far the largest tribe in Israel with 74,600 soldiers. Dan is second-most with 62,700, which is much closer to the average values for the tribes (between 40,000 and 60,000). The smallest tribes are Manasseh and Benjamin. Manasseh grows to become a significant military force, but Benjamin will remain one of the smaller tribes for a long time, though it does play a significant role in Israeli politics, as we will later see. The second-most militarily significant tribe in the future is Ephraim, but that doesn't really show through here as their tribal population is smaller than average.
Note that the ordering of the tribes in the census is chronological to start, but then swiftly breaks down into a seemingly random order. Reuben and Simeon are the first two sons of Jacob, but Levi is missing and replaced with Gad, while Judah takes the fourth position (Judah is the fourth-born). After that, I don't see any particular ordering, whether chronological or grouped by mother.
These results are more interesting when we compare them with the second census that is taken later in Numbers, so I will come back and discuss it more then.
The last thing I will discuss is the exemption of the Levites and their role as the custodians of the tabernacle. We already knew going back to Ex 32:29 that the Levites were "[dedicated]... to the LORD", and I kind of hinted at things when discussing the title of Leviticus, even though that's more of a reference to the Levitical priesthood through Aaron than the quasi-priestly tribe of Levi. We saw another veiled reference to the special role of the Levites in Leviticus 25:32-34 which says that these cities are the tribal inheritance of Levi. Now we see the role of Levi is to be a not-quite-priest. The Levites are to take responsibility for the tabernacle, disassembling it and moving it from place to place as they wander through the desert. This helps resolve the quandary of how the tabernacle could be moved with the people: there are too few priests to do it (Aaron has only two sons right now), and the people are not consecrated that they might touch the sacred things. So the answer is that the Levites will be specifically dedicated to carrying the tabernacle.
More generally, the Levites are semi-professional helpers for the priests. As we saw in Lev 25:34, the Levitical cities have assigned pasture land around them, with the expectation that the Levites will spend some time farming or pasturing in that land and some time assisting the priests. Yet, the very fact that they have cities means that the nomadic migrations were over, so the Levites would have to find some new set of duties if they are still assigned to helping the priests. This will be discussed later in the OT. For now, all we know is that the Levites are to form a sort of boundary around the tabernacle, protecting the sons of Israel from "wrath on the congregation".
The reason the Levites weren't counted in the census is that they would not have been expected to fight. Their assignment to the tabernacle probably demands that they stay in the camp. The Levites are not priests, however. As we read later, the Levites do not have permission to perform nearly any of the priestly ordinances, but to my mind this still calls up images of the "royal priesthood, a holy nation" (Ex 19:6), which I made a big deal about back when I first discussed it. This is the first tribe that can operate in a semi-priestly fashion, so it strikes me as a significant step in the overall plan of redemption.
Numbers will talk more about the Levitical assignment in a little while. For now, I will move on to chapter 2 and the arrangement of the camp.
In this case, the twelve tribes are the ten sons of Jacob, plus two sons of Joseph, excluding Levi. This is the standard formulation for most of the OT, which is a bit ironic to me how the "twelve sons of Jacob" are not the same as the "twelve tribes": Joseph and Levi are replaced by Ephraim and Manasseh in the tribal lists. These listings are usually pretty confusing in Exodus because Levi wasn't exactly "elevated" to their pseudo-priestly role at the time, yet the author will almost always insist on counting twelve tribes/sons of Jacob for symbolic reasons (twelve being the number of governance). For instance, the prophecies of Jacob in Gen 49 include Joseph and Levi, without mentioning the sons of Joseph. Ex 1:2-4 lists the eleven sons of Jacob, plus Joseph who was already in Egypt. Ex 28:21 and Ex 39:14 mention twelve gemstones in the priestly garments that symbolize the twelve tribes without specifying which twelve tribes are intended. Given its place in the story, it probably means to include Joseph and Levi rather than Ephraim and Manasseh. Yet now, going forward it will be difficult to ascertain which list of tribes is intended. In most cases it is the Ephraim/Manasseh formulation, because those are the tribes which are granted a tribal inheritance of land, so they become increasingly significant compared to Joseph/Levi. Also consider the peculiar listing in Revelation 7:5-8 which includes Levi and Manasseh, excluding Joseph and Dan (?!).