16 The verb (פָּקַד, paqad) means “to visit, appoint, muster, number.” The word is a common one in scripture. It has as its basic meaning the idea of “determining the destiny” of someone, by appointing, mustering, or visiting. When God “visits,” it is a divine intervention for either blessing or cursing. Here it is the taking of a census for war (see G. André, Determining the Destiny [], 16).
17 The construction uses the participle “going out” followed by the noun “army.” It describes everyone “going out in a military group,” meaning serving in the army. It was the duty of every able-bodied Israelite to serve in this “peoples” army. There were probably exemptions for the infirm or the crippled, but every male over twenty was chosen. For a discussion of warfare, see P. C. Craigie, The Problem of War in the Old Testament, and P. D. Miller, “The Divine Council and the Prophetic Call to War,” 18 (1968): 100–107.
18 The text simply has “from twenty years old and higher.”
19 Heb “and up.”
20 The noun (צָבָא, tsava’) means “army” or “military group.” But the word can also be used for nonmilitary divisions of labor (Num 4:3).

In other versions

From twenty years old and upward, all in Israel who are able to go to war, you and Aaron shall list them, company by company.

You and Aaron are to count according to their divisions all the men in Israel who are twenty years old or more and able to serve in the army.

You and Aaron are to register those who are twenty years old or more by their military divisions—everyone who can serve in Israel’s army.

twenty years old or older who are able to go to war. You and Aaron must register the troops,

from twenty years old and above, everyone in Israel who is able to go to war. You and Aaron must muster them for their wars.

From twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies.

Public Domain