The usage of “who knows” [duplicate]

Is the second sentence correct English? What is the grammatical role and meaning of “who knows how many jobs”?

…Traditional cars happen to be human sized to transport humans but tiny autos can work in wear houses [sic] and gigantic autos can work in pit mines. Moving stuff around is who knows how many jobs but the transportation industry employs about three million people.

Source: “Humans Need Not Apply”

Answer

“Who-knows-how-many” is used as a substitute for some number. It is an example of a placeholder noun or word, or in this case, noun phrase

I have read 7 books.

I have read who-knows-how-many books.

Attribution
Source : Link , Question Author : sarah , Answer Author : Community

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