Don't you just hate God-assisted holy war massacres? I do. They're so damned boring.
But we might as well get used to them because God likes them. A lot. There are about 50 holy war massacres on the list of God's 158 killings in the Bible. So there's just no way to avoid them.
Here's what happened in this one.
King Arad heard that the Israelites were coming, so he fought against them when they tried to invade his land.
When king Arad the Canaanite, which dwelt in the south, heard tell that Israel came by the way of the spies; then he fought against Israel, and took some of them prisoners. Numbers 21:1
So the Israelites asked God for help, promising to kill everyone in several cities.
Israel vowed a vow unto the LORD, and said, If thou wilt indeed deliver this people into my hand, then I will utterly destroy their cities. 21:2
God didn't have to think about this one. Here's how he responded.
The LORD hearkened to the voice of Israel, and delivered up the Canaanites. 21:3a
So the Israelites, with God's help, killed everyone in several Canaanite cities.
And they utterly destroyed them and their cities. 21:3b
I'm glad we got this one over with. It was pretty boring, wasn't it?
God's next killing will be better, I promise.
The Bible doesn't tell us the population of the Aradite cities. So I guessed there were 3 cities, each with 1000 people, for a total of 3000 victims.