Tafsir of Al-Waqi'ah 56:13

(A multitude of the former generations)

This is the predicate of an implied subject, meaning: "They are a multitude," etc. It is also permissible for it to be a subject whose predicate is omitted, meaning: "Among them is a multitude." It may also be a first or second predicate for the [pronoun] "those" (ula'ika). Abu al-Baqa’ permitted it to be a subject, with the predicate being (on thrones).

A thullah (multitude), according to the well-known opinion, is a group, whether large or small. Al-Zamakhshari said: "It is a large group of people," and he cited as evidence the verse: And there came to them a 'thullah' (multitude) of Khandaf—[followed by] with an army like a surging torrent of foam. The Almighty's saying thereafter: (And a few), etc., is sufficient evidence for [it meaning] a large number.

The apparent implication is that he cited the verse as evidence for the meaning of "largeness" in the word thullah. If the letter ba in the verse is tajridiyyah (expletive/intensive), then it is an explicit text; otherwise, the inference [that it implies a large number] is based on the fact that the context is one of hyperbole and praise. As for his inference from what follows [the verse], that is because contrast is required, since by consensus, the word thullah was not coined for a "few," so as to interpret what follows as [a matter of] stylistic variation. Rather, it is either for a large number—and its etymology supports this more strongly, as thall means pouring, and it also means demolishing entirely, and thillah (with a kasra) refers to a large flock of sheep—or it is for a group in the absolute sense, like a faction or a piece, from thall meaning to break, as if it were a group broken or cut off from the people. However, usage has come to predominate for the "large number." Thus, the meaning is: a large group of the "former generations," who are the people who preceded from the time of Adam to our Prophet—peace and blessings be upon them both—and upon the great prophets between them.