Before certain numbers
The article a can be substituted for one with certain numbers:
e.g.

one hundred = a hundred

one thousand = a thousand

one hundred thousand (100,000) = a hundred thousand

one million (1,000,000) = a million

one million and a half (1,500,000) = a million and a half

There are also certain idiomatic numerical noun expressions in English which take the article a.
e.g.

a couple (two)

a hat-trick (three goals scored by the same player in a football match)

a dozen (twelve)

a baker's dozen (thirteen)

a score (twenty)

a gross (144)

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I have to buy a few souvenirs before I leave. (three or four)

You can use few  with a plural noun without the article a, but there is a change in meaning:
e.g.

Few people can survive for long without water. (most people cannot)

The expression a few of  is used to refer to a small number of a specific grouping. In this case the article the, a possessive pronoun, a possessive noun or another determiner is used after of,
e.g.:

A few of the men in my office have started a darts club.

A few of my neighbour's carp died during the hot summer.

Would you like a few of these chocolates?

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A great many houses in Kobe were destroyed by the earthquake.

A great many scientists attended the international conference.

Note we cannot say:

??? a many houses ???

??? a many scientists ???

If you want to be specific or to show previous mention of the plural noun, add the word of after great many and the article the:

e.g.

A great many of the houses destroyed in the earthquake were old wooden structures.

A lot of newer property was also badly damaged by the earthquake. A great many of the buildings had to be demolished.

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