Albert Barnes Commentary Isaiah 21:17

"and the residue of the number of the archers, the mighty men of the children of Kedar, shall be few; for Jehovah, the God of Israel, hath spoken it." — Isaiah 21:17 (ASV)

And the residue of the number - That is, those who will be left in the invasion. Or perhaps it may be read, ‘There will be a remnant of the number of bowmen; the mighty people of Kedar will be diminished.’

Of archers - Hebrew, ‘Of the bow;’ that is, of those who use bows in war. The bow was the common instrument in hunting and in war among the ancients.

Shall be diminished - Hebrew, ‘Will be made small;’ they will be reduced to a very small number. We cannot indeed determine the precise historical event to which this refers, but the whole connection and circumstances seem to make it probable that it referred to the invasion by the Assyrian when he went up against Judah, or when he was on his way to Egypt.