The noun phrase
NTRODUCTION
Structure
Noun phrases consist minimally of a noun, which acts as the head of the noun phrase. The head may be accompanied by dependent elements before or after it. The followig are examples of noun phrases; heads are in green, words in black are dependent elements:
music
summer
we
him
a dog
the old man
that table in the corner
a nice day at the beach
the sofa we bought in the sale
(Cambridge Grammar of English, ditto #4)
A3 NOUN PHRASES AND DETERMINERS
A3.1 Noun phrases
In A2 we looked at nouns as a word class. But we want to study texts and analyse sentences (for example, to identify subjects and objects), we need to recognise a larger unit: the noun phrase. A noun phrase is a noun and all the words that 'go' with it. It can consist of just a noun:
Money is bad for you.
People are strange.
London is a fantastic place.
And a pronoun can also function as a noun phrase:
She is my best friend.
But usually there is more than one word.
Noun phrases can consist of up to four parts, as in the diagram:
DETERMINER......PREMODIFIER....... HEAD........ POSTMODIFIER
all the ...............tall..................... girls.......... with red hair
Figure A3. 1.1 The four parts of a noun phrase
... omitted ...
Heads
The head is the central part of a noun phrase; it is the only part which is obligatory, though if it is a singular count noun, there must be a determiner with it: a table or that table, not simply 'table'. Heads are usually nouns, but can sometimes be adjectives
(the poor); see B3. The head is the word that changes for number. It agrees with the determiner and any following verb (if the noun phrase is the subject).
(ENGLISH GRAMMAR, Roger Berry, Routledge)
I suppose the sources say countable nouns are not (noun) phrases on their own.