Numbers 1:3, 18, 20-21

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

and they assembled the whole community of Israel on that very day. All the people were registered according to their ancestry by their clans and families. The men of Israel who were twenty years old or older were listed one by one

This is the number of men twenty years old or older who were able to go to war, as their names were listed in the records of their clans and families