"The farthest reach" or "the farthest reaches"?

Question

I heard on TV (an American science program, forgot the name of the program):

We will venture out into the farthest reaches of the cosmos.

It makes me wonder if/when the noun "reach" is a count noun and can be pluralized. Bizarrely, according to Macmillan Dictionary and Cambridge Dictionary, it is either a noncount noun or a singular noun.

Google shows both terms are in use. For example, there is a book called Farthest Reach, but also instances of books and songs by the title (the) Farthest Reaches abound.

Google Ngram shows something interesting. The popularity of these two terms has changed over time:

So is reach also a count noun? And when should it be pluralized?

Answer 1

When reach refers to an extent, as in the reach of his arm or the reach of this weapon, it is countable and usually singular but can be used in the plural:

What is the reach of this bow you're making out of carbon fiber, compared to a bow of the same weight made from yew?

Let Larry try to get that can down from the back of that high shelf. He has the farthest reach.

Great swimmers often have long reaches.

Great swimmers often have a long reach.

When the meaning refers to a distant or remote place, whether literally or figuratively, it is typically in the plural:

They traveled to the farthest reaches of the taiga.

They sought to explore the farthest reaches of the human mind.

Answer 2

The pluralirity hinges on dimensionality. Space has directions (up, down, left, right, and many many more depending on how muxh credibility you put in string theory). The reach of a bow or swimmer is in one direction so won't be pluralised. Space; however, requires reaches into multiple directions/dimensions in order to span its coverage and so will be used pluralised (at least in a scientific or just scifi context where such thinking will dominate over the definition of a noun).