What does it mean to end a sentence with ひとつだけ?

Question

I came across a sentence that ends with ひとつだけ in a book and searching on jisho gave me zero results (https://jisho.org/search/ひとつだけ); nor did searching for the individual parsing of what I remember from the audio of the VN (i.e. hitotsu--DakE) give me any helpful results that would make sense in context.

しかも、出入り口は錆の浮いた鉄扉がひとつだけ。

TDLR: What does the sentence end(?) ひとつだけ mean in the above context and does it have a general meaning when used at the end of a sentence as opposed to being used elsewhere in the sentence?

Edit:

Context: Narrator is describing a unfamiliar room he wakes up in. I think that ひとつだけ in the above context will not work as individual parsings as ひとつ most likely means either the "adverbial noun – just" or the numeric one in the above context that adverbially/numerically describes the 鉄扉 (iron door). The next bit, だけ most likely is a particle ("only") but I don't see how the "only" can affect the 鉄扉 as the ひとつ is already doing that to the "iron door". The only way I can see both affecting 鉄扉 is if ひとつだけ is a qualifier sentence (technically a qualifier phrase) being used in reverse-order.

Answer 1

Consider these simpler examples:

• リンゴは2個です。
  The number of apples is two.
• 参加者は3人です。
  The number of participants is three.
• 「箱の中には何がありますか？」「バナナが2本です。」
  "What is in the box?" "Two bananas."

Although "Apples are two" makes no sense in English, リンゴは2個です is a perfectly valid way to tell the number of things in Japanese. As you already know, the topic marker は is used for things already in the universe of discourse. が is used to introduce a new item into the universe of discourse.

You can use both は and が at the same time:

参加者は日本人が3人です。
Regarding (the number of) participants, there are three Japanese people.

You can drop です/だ to make a sentence look more compact and/or dramatic (体言止め):

参加者は3人。
(There are) three participants.

You can add だけ (or のみ) to add the meaning of "only".

参加者は3人だけです。
The number of participants is only three.

Combined together, we have something like:

参加者は日本人が3人だけ。
Regarding participants, there are only three Japanese people.

This is exactly how your sentence in question is constructed, too.