"No one's left in his rack" is effectively "plural," because it covers everybody.

In your (less idiomatic examples), "racks" is the only logical choice.

Not to confuse things further, but a "rack" covers all forms of sleeping structures, including bunks and beds.

"Hit the rack" is a familiar U.S. expression.

(It used to be "hit the sack," but many people got tired of saying that.)

If "the rack monster's got you," you're so tired (as from studying all night) you either just have to go to bed or else you can't get out of it.

"Two nations divided by a common language."