Albert Barnes Commentary Amos 5:3

"For thus saith the Lord Jehovah: The city that went forth a thousand shall have a hundred left, and that which went forth a hundred shall have ten left, to the house of Israel." — Amos 5:3 (ASV)

The city that went out by a thousand - (that is, probably one that sent out a thousand fighting men, as the phrase "went out" is often used to mean "went out to fight"), shall have, literally "shall retain," an hundred. She was to be decimated.

Only, the tenth alone was to be reserved alive; the nine-tenths were to be destroyed. And this, in large and small places alike. The city that went forth an hundred shall retain ten. Smaller places escape because of their obscurity, larger ones because of their strength and situation. One common doom was to befall all. Out of all that multitude, only one tithe was to be preserved, "dedicated to God," that remnant which God always promised to reserve.