The book of Numbers is about two censuses that were taken; one at the beginning of the book and one at the end of the book. In 1:1 the Lord commands Moses to number the people. It seems to take approximately 19 days to do that. The object of this exercise was to list the numbers of men twenty years of age and above who were “able to go to war” (v. 3). This indicates its military purpose. The God of Israel was mustering His people for advance into the Promised Land and into the fulfillment of His sovereign purposes for them.
In English we use the word “count” in many different ways. You can be a person who is a “No account.” If you’re a rocket you can have a “count down.” If you’re a boxer you can be “counted out.” It’s even used sometimes to refer to someone’s position like “Count Dracula.” We use it to express our willingness to be part of something; “count me in,” or to express our desire not to be part of something; “count me out.” Those in the military know what it’s like to “count off.” It’s to call out your presence. I think this might be the best way to understand the “counting” in the book of Numbers. It’s about those who are ready, willing, and able to do what needs to be done. In this case, it’s numbering those capable and willing bodies, who will go forward to the fulfillment of God’s purposes for their lives. What God is numbering in this book is those who can be “counted on.”