Question

He goes to a concert once a month.
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In this sentence what does "a" before 'concert' mean?
Does 'a' mean 'one'? Or is 'a concert' used in generic use?

Answer 1

It's rather generic, but yes, it does mean "one." If he went to more than one concert a month, you'd use one of these:

He goes to at least one concert a month.
He goes to at least two concerts a month.
He goes to a couple of concerts each month.
He goes to as many concerts as he can every month.
He goes to several concerts a month.
(And there are others or you can alternate the wording of those above.)