On this page, it says that "plus de" is an adverb of quantity.
That page links to this page which tells me more information about "plus de".
There is a section called "Comparing Nouns". Here are some examples it gives:
J'ai plus d'idées que toi. (Comparing a noun between two subjects)
I have more ideas than you [have ideas].
Il y a moins de pommes que d'oranges. (Comparing two nouns that are objects)
There are fewer apples than oranges.
When "plus de" is used to compare nouns, as in the examples above, is "plus de" still considered to be an adverb of quantity? I had thought it would instead be considered an adjective, because adjectives modify nouns?
If I want to say "more [noun]", in a way that does not compare, as in:
I am a poor student. I want more money.
Can I use "plus de"? The page I linked to doesn't seem to suggest that I can.