What pluralizes the word "iwa" here

よく岩に潰されなかったものだ

…まるで岩をすり抜けたとしかよく岩に潰されなかったものだ

I don't see anything that indicates that there were "rockS." Of course I need to know the background - whether there was one big piece of boulder rolling down a narrow mountain path, or bunch of rocks rolling down the hill, for example.

The only clue here is the phrase "iwa wo surinuketa." "Surinukeru" in the strict sense means brushing by, but somehow managing to get through, so in my mind there is a picture of somehow narrowly dodging rocks left and right. But the write might have just meant "escaped" by using this word.

Did you find that sentence from NARUTO? (manga?)
https://sakuramanga.net/wp-content/uploads/Naruto/63_079.jpg
http://d.hatena.ne.jp/naruto_AG23/20120924/p1
http://narutoxjunkie.blog107.fc2.com/blog-entry-910.html

That sentence emphasizes
"I was very surprised at the fact that it(he = Uchiha Obito) was not crushed by a/the rock(s)",
so neither the number of rocks nor the size is a main point.
The contents of the question are different,
but I was surprised that the question of the same sentence was already posted on this forum.
https://www.japan-guide.com/forum/quereadisplay.html?0+105048
(Please copy correctly.)
When reading the guess of this scene, it is not known who "I(= elder?)" is.
The second site blogger guess he is "Uchiha Madara".

BUT, which of the following is your question?
1) Does this 岩 mean plural form?
2) Does 岩 have plural form?
3) How to say/write in plural form if so?

Anyway, in Japanese, Noun words
that have the same way of writing as "singular" in "plural form" like English also many.
In the case of "(moving) rock(s)", it can be regarded as "organism (also like human = 擬人化)"
and attached with "-tachi(-達)" to be "岩達(iwa-tachi)", but just using possible, not always.
https://en.wiktionary.org/wiki/%E9%81%94
One elementary level "友達(tomo-dachi) = a friend/frends" is also a word
that does not distinguish btw singular and plural.
If you do not understand the meaning of this kanji(= 達),
you may not be able to guess whether "-tachi" and "-dachi" are the same.
"岩" like many nouns, plural forms are also written in singular form usual,
but "友達" is oppsite it is one of the words to write in plural form even in the singular form.

In "友達" case can be written as only "友(tomo)" like "岩",
but the case of writing in singular is not so many in current Japanese.
If the target is human or anthropomorphic characters,
"友人(yuu-jin)" can also be used, but this can not be used for "岩".

And "岩" is a thing which does not move originally,
but "-達" is also attached at the time of the state
where the moving object is stopped, or "inorganic matter".

In the case of living things,
"-等(-ra)" and "-共(-domo)" can also be used instead of "-達",
https://en.wiktionary.org/wiki/%E7%AD%89
https://en.wiktionary.org/wiki/%E4%BE%9B
but it is easy to express roughly and do not use for "岩" even moving.
岩 is not a living thing mentioned already, so do not use both usual.
Like "岩らへん(-ra-hen)", youngers use words to point to places, positions, markers,
this "へん(= 辺)" means "around", あの岩らへん = around that rock(s).
(In this case we use Hiragana instead of Kanji.)
https://en.wiktionary.org/wiki/%E8%BE%BA
https://www.nhk.or.jp/bunken/research/kotoba/20150201_3.html