"Count the heads…from twenty years old and up, everyone who goes out in the (tzava) army in Israel…" Tzava is the modern Hebrew word for army and how most understand it in this verse. However, one is fit for military service at an even younger age and indeed some modern armies accept recruits at sixteen. The Ramban explains that the word tzava can also mean an array or gathering. This is how the word first appears in Genesis: The heavens and earth were completed and all their (tzavaam) array.Thus, what we have here is not a military census but a general census.