The word caravan is a noun. The plural is caravans.
I would guess it is a figure of speech relating to a caravan that is so long that it is not practical to count the number of individual members.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
what part of speech is beneath
adverb
No such part
The part of speech for "explicit" is an adjective.
The part of speech for "lecturer" is a noun.
The part of speech for "rapid" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "breezy" is an adjective.