For now men leaped from the ships to the quays of the Harlond and swept north like a storm. There came Legolas, and Gimli wielding his axe, and Halbarad with the standard, and Elladan and Elrohir with stars on their brow, and the dour-handed Dúnedain, Rangers of the North, leading a great valour of the folk of Lebennin and Lamedon and the fiefs of the South. But before all went Aragorn with the Flame of the West, Andúril like a new fire kindled, Narsil re-forged as deadly as of old: and upon his brow was the Star of Elendil. It’s the phrase “a great valour of the folk of Lebennin [etc.]” that attracted my notice. This usage struck me as a bit unusual. Here, valour looks like it might be intended as a collective noun, like a gaggle of geese, a skulk of foxes, a swarm of bees, etc. It’s possible to read it differently, of course — valour doesn’t have to be a collective term. But whatever the case, the phrasing is a bit outside normal English usage. The word valour is seldom used with the indefinite article. I did a little poking around and haven’t been able to find an example of the phrasing, “a valour of <plural noun>
There are reasons to suppose it might be intended as a collective phrase. Elsewhere in The Lord of the Rings, Tolkien uses a very similar construction many times. A few quick examples will suffice to make the point: (1) “a great following of Hobbits”, (2) “a great expanse of years”, (3) “a great troop of Orcs”, (4) “a great host of men”, (5) “a great cavalry of horsemen”, (6) “a great concourse of trumpets”, (7) “a great company of hill-trolls out of Gorgoroth”, (8) “a great welter of cloud and smoke”, and many others.
Colorful collective nouns for groups of animals and people go back to the middle of the 15th century. Many of these are first recorded in Lydgate’s Hors, Shepe, & the Ghoos (c. 1470); others, in The Boke of St. Albans (c. 1480). These early sporting and hawking terms brought us an unkindness of ravens, a charm of goldfinches, a parliament of owls, a knot of toads. Later, the idea was extended to people — a pity of prisoners, a hastiness of cooks, and so on. These are wonderfully imaginative, so it’s no wonder that people have continued to coin new ones in all the centuries since. We now have the likes of a murder of crows, a frenzy of sharks, an unction of undertakers, a blur of impressionists, and — one of my favorites — a shrivel of critics. 
I’m not sure whether Tolkien intended to coin a new collective noun, but doesn’t a valour of knights sound perfect? (Prior to this post, the exact phrase “a valour of knights” returned zero results from Google. Of course, that will no longer be true once this post makes it into their indexes.)
 Salu, Mary and Robert T. Farrell. J.R.R. Tolkien, Scholar and Storyteller: Essays in Memoriam. Ithaca: Cornell University Press, 1979, p. 17.
 RK, p. 123.
 For more, see Lipton, James. An Exaltation of Larks: The Ultimate Edition. New York: Penguin, 1993.