Number modifiers after a noun

I have a question regarding numerical adjectives. Admittedly, I have not done any extensive research on this, but I seem to be having a difficult time finding it readily available in most of my grammar books. There is plenty of information regarding numbers, modifiers and adjectives.

A student believed that I was wrong and I was not really sure. I had never considered the point. I believe I am correct, but I can not explain why. I was not able to give a legitimate enough answer to her either; aside from an overly complicated guess regarding the difference between ordinal and cardinal placement.

In the prepositional phrase, 'on page 21', the noun is 'page' and the adjective is '21'.

Am I wrong? Honestly, when the phrase is drawn out completly, 'on the page number 21', it seems even more confusing as to what the parts of speech are. '21' seems to be modifying the page because that can change, 23, 26, 27, etc. It is not a quantifier either, just a descriptive adjective. Quantifiers have to go in the front.

So, what is '21'? And, what is 'number' in the second example? Is that modifying '21'? They have to be adjectives, but why are they at the end when most are in the front.

It is easy to determine what the parts of speech are for 'on the page'. Another simpler and much more obvious example would be, 'on the 21st page'.

I hope I have explained myself well enough. Thank you for your time.

With respect,

Timm

PS:

Also, I would like some recommendations on grammar texts. The main one I have

(Longman; Leech/Cunningham/Ivanic) is not that thorough although it is claimed to be the standard. Maybe I just overlooked the answer in the book. I have other texts, but they have even more limited explanations.