What is the best way to get the total number of pages in a document and insert in the document?

[Andrew]

@Bryn

This question and answer book contains 
#context[#counter(page).final().at(0)] pages

Is this the best way to generate this text, or is there a more natural alternative which I am overlooking?

Yes, but you can optimize/simplify it like this:

#context counter(page).final().first()

@Bryn

Also, not that I understand the concept of context particularly, but the documentation for Context starts with

Sometimes, we want to create content that reacts to its location in the document.

But in this case, I would have assumed that the total number of pages in the document is not contingent on the location in the document. What am I missing here?

The key part here is “sometimes”. This does not apply to your example, but does for this:

#context here().position()