Above: an artist's depiction of Moses.
In the Book of Numbers, we start getting into the logistics of the Israelites' stay in the Sinai Desert.
Most scholars suspect their numbers to have been fudged, but the book is about how more than a million people (their census counted only males, which numbered 600,000) managed to live and wander about in the desert. But think about this, in the beginning of Exodus, the Israelites numbered 70, and in the course of 400 years, their population increased to well over a million people (based on a census of 600,000 males).
There are other censuses taken, like of all male children, or of all males 30 to 50 years old. These were done for different administrative purposes.
The tribe of the Levites, however, were not included in this census. They were the designated holy tribe who would supervise the building up and tearing down of the Tabernacle, and also see to its security, maintenance, and of course the teachings of Yahweh.
Remember Aaron's sons, the ones who God killed because they offered an unauthorized incense to Him? Well, Numbers Chapter 3 mentions them again, but it's basically a shortened version of the same story.
An interesting development in Chapter 4, God determines that Gershonite and Merari males, ages 30 to 50 years old shall do all the hard labor. The Gershonites and Merari are families within the Levite tribe, who had just been designated to oversee the Tabernacle.
Imagine being determined to do all the backbreaking hard labor of building up and tearing down the Tabernacle every day? They had to worry about special curtains, pillars, setting up the building and sacrificial altar correctly, covering certain holy items, building and rebuilding the fence around the Tabernacle, setting up offering plates, cleaning up any mess from sacrifices, etc.
Next time, God starts yet another crusade against - you guessed it - lepers!