A number, a determiner, tells us how many items are in the set. In
this sense, it is a modifier. When the number is two or above, the
noun takes the plural form. [Note: modifiers can be nouns OR adjectives]
The book has two hundred pages
Bruce's workout lasted five minutes.
grammar quizzes
Numbers: one, two, three Numbers such as one, five, eleven, two
hundred are cardinal numbers. We most commonly use cardinal numbers as
determiners (before nouns). When we use them in this way, we can use
other determiners such as articles (a/an, the) and possessives (my,
your) in front of them. We can use cardinal numbers + of before
determiners (one of my friends):
She loves animals and has two dogs, three cats and one rabbit.
Cambridge Dictionary
Some theories of grammar do not include determiners as a part of speech and consider "two" in this example [two hats] to be an adjective.
Wikipedia
Conclusion for purposes of learning English:
So, that means that the word "*approximately" in "approximately five people" (in sentences or as an answer in speech) can indeed be seen as an adverb qualifying the modifier or adjective "five" (the cardinal number).
Reminder: Oxford Dictionaries via Google:
adverbs
a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree, etc. (e.g., gently, quite, then, there ).
For example:
- We drove approximately ten miles. [modifies the phrase "ten miles" or the number "ten"] This can be viewed two ways. But it is still an adverb as per the definition.
- He only does the task approximately, not thoroughly. [modifies the verb do]
Answer; Yes, "approximately" is an adverb modifying the cardinal number used as an adjective.