<PERSON> probably only needs one eye if he has enough time and can make sufficiently accurate spectral measurements.
First, note that <PERSON> was watching on a sunny day; we'll assume that between incident intensity and albedo that object were reflecting on the order of $ 100 \mathrm{W}/\mathrm{m}^2$ light, which is about $10^{22}$ photons per second. At 24 kilometers, that's down to about $10^8$ photons per $\mathrm{cm}^2$.
We're not sure how big <PERSON>' eyes are, as the books don't say, but we can assume they're not freakishly huge so are on the order of 1 cm in diameter, which gives him about $6 \cdot 10^-5$ radians angular resolution, or roughly $1.5 \mathrm{m}$. As already described, this ought to be adequate to count the number of riders.
Now there are two factors which are hugely important. First, the riders are moving. Thus, by looking at temporal correlations in spectra, Legolas can in principle deduce what the spectra of riders are distinct from the background. 840
We can also assume that he's familiar with the spectra of various common objects (leather, hair of various colors, and so on). He thus can make a sub-resolution mixture model where he hypothesizes $n$ objects of distinct spectra and tries to find the size/luminance of each. This is probably the trickiest part, as the spectra of many items tend to be rather broad, giving substantial overlap in spectra. Let's suppose that the object he's looking for has only a 10% difference in spectral profile from the others (in aggregate). Then with a one-second integration time he'd have photon shot noise on the order of $10^4$ photons but a signal of about $A\cdot10^7$ photons where $A$ is the fractional luminance of the target object within the diffraction-limited field of view.
Since super-resolution microscopy can resolve items approximately proportional to the SNR (simplest example: if a source is all in one pixel, all in another, or a fraction in between, you basically just have to compare intensity in those two pixels), this means that <PERSON> could potentially find a bright object to within on the order of $1.5 \mathrm{mm}$. If he uses the gleam off a helmet and stirrup, for instance, he could measure height adequately well and pick out details like "yellow is their hair". 675