Learn Spanish (Latin America): Una cuarentena de manifestantes iba a caminar hasta allí. - About forty protesters were going to walk over there.

It does! However, here it is acting as a collective approximate numeral. By attaching the -ena suffix to cuarenta (forty), it forms a noun meaning "a group of about forty." This is why it translates to "about forty" or "some forty" protesters.

Why is the verb singular (iba) when there are multiple protesters (manifestantes)?

The grammatical subject of the sentence is the singular noun phrase una cuarentena, so the verb takes the singular form iba. While you will sometimes hear native speakers use the plural iban to match the plural manifestantes (known as agreement by sense), using the singular iba is grammatically correct and very common.

Why use hasta allí instead of just saying they were going to walk hacia allí?

Hasta translates to "until" or "as far as." Using hasta allí emphasizes the specific destination they intended to reach ("all the way over there"). By contrast, hacia allí would just mean "toward there," focusing on the direction they were walking rather than the final destination.

Why use the imperfect tense (iba a caminar) instead of the preterite (fue a caminar)?