The Numbers 3:16

16 So Moses counted them, as he was commanded by the word of the LORD.



So in Leviticus God says to sacrifice animals in order to make up for sin and remeber God, but who was going to run the operation. Well, God said that every firstborn son should dedicated to him and serve him. But logistically, that's a little tough to coordinate, so God declares that the clan of the Levites can take the place of the firstborns. But are there enough Levites to take the place of all of the firstborns? Thus we have the need for a census and a count of all the people (hence "Numbers").

What was the result? Well it turns out that there were about 270 more firstborns then Levites and there were sacrifices offed to make up for the difference. While in Levitcus you see alot of the qualitative detail given to his following, here in numbers you see much of the quantitative precision. A ton of care was given to following God to the letter back then.