This question already has an answer here:
The indefinite article of English takes the two forms a and an. Semantically, they can be regarded as meaning "one", usually without emphasis. They can be used only with singular countable nouns; for the possible use of some (or any) as an equivalent with plural and uncountable nouns, see Use of some below.
It says if a noun comes with an indefinite article, the noun is singular and countable.
Then there are phrases such as 'a lot of', 'a number of', and 'once upon a time'. Are 'lot', 'number', 'time' above countable nouns as well? I have never heard of people saying 'two numbers of people' or 'once upon three times', which makes me doubt that they are actually countable.