Chapter 1

The book starts with a timestamp:

1 The LORD spoke to Moses in the wilderness of Sinai, in the tent of meeting, on the first day of the second month, in the second year after they had come out of the land of Egypt, saying, [Num 1:1 ESV]

So they were camped at Sinai to this point, 13 months now, roughly.

God orders Moses to take a census of all men of Israel, 20 years old and upward.  It is to be done by tribe and by clan.  The head of each tribe - by God's own appointment, is to assist with the census.  There were 603,550, and these were all considered capable of going to war.  This was their maximum army size.

Levites not listed.  They were here put in charge of all things related to the tabernacle - setting it up, taking it down, and carrying it.  Also guarding it - by camping all around it - so that no one who might be unclean or unworthy to approach it, or who might not understand all the rules associated with it, and so be killed.  Remember that the Levites - Moses' tribe - were set apart after they went through the camp killing their brothers and so on after the golden calf incident.  Only the Levites did that - only they were loyal enough to move over to Moses' side when he called.  This is their reward.