When does "shadow" become plural?

Solution 1:

Generally, you can use your discretion in deciding whether to consider a silhouetted space a singular shadow or multiple shadows. You might consider thinking of it like the word "shape," which likewise can apply with different levels of granularity. You could say, "Look at the shape of the palm trees," referring to the shape of the cluster of trees as a whole, or you could say "Look at the shapes of the palm trees." You could even describe a singular object as having multiple shapes or shadows.

I looked at the shadow of the tree.

I looked at the shadows of the tree.

It's also worth noting, however, that "shadows" plural can carry a meaning distinct from "shadow" singular, referring to darkness generally. Merriam-Webster provides this definition:

7 shadows plural : dark 1a

Consider as an example this figurative use in a headline from The New York Times:

Once in the Shadows, Europe’s Neo-Fascists Are Re-emerging

Solution 2:

'Shadow' is plural when you think it is.

This really isn't as difficult or unusual as you're making it out to be. Think of a much more common and intuitive example of the same idea: hair.

It's not grammatically incorrect to say

Her hairs are long.

but syntactically it focuses on the individual hairs in a very uncommon way. Native English speakers don't usually think of hairs as individuals, so we speak of them as a mass:

His hair is long.

For the most part, English considers things that are easily counted as countable and stuff that isn't as uncountable. If the shadows are discrete, it would be natural to speak of "the shadows" and if they formed a single mass shadow, it would be natural to speak of "the shadow" just as all the world's inlets, seas, and oceans coalesce into "the sea" or "the ocean". To native speakers,

The crowd gathered in the shadow of the palm trees.

and

The crowd gathered in the shadows of the palm trees.

are both fine but mean different things.

The first creates a very definite sense that the trees' canopy is more clustered and full, casting a single great shadow upon the ground. The second creates the opposite sense that the shadows are more discrete, either because the trees are separate enough to form no canopy at all and people gather separately under separate shadows or because the mass shadow is spotty in some way that makes the individual shadows noticeable. (In my own mind's eye, I still see a single canopy, but with a sinking sun causing the shadows of the separate palm trunks to be more noticeable; other readers would probably have different images but the point stands.)

If it helps, the base definition of shadow at the OED is not "the darkened area created by obstructed sunlight". It's

Comparative darkness.

Just like water, sand, hair, flour, and darkness, it defaults to being uncountable. It only feels different because it's so much easier to think of separate discrete instances of shadow. But it's just like a restaurant offering to bring four [glasses of] waters, Arabia having its [several patches of different] sands, the [instances of] hairs growing on someone's nose, a store selling four [brands of] flours, or the [species of] darknesses which are ignorance, unkindness, and incuriosity. They can still be plural when it makes sense for them to be discussed as discrete examples or types of the mass they usually represent.