The speech in Our Town in which the Stage Manager/Minister mentions "M marries N" is in a sort of soliloquy for the Stage Manager in which he reveals personal thoughts and impressions. We know he is revealing personal thoughts because he says, "Do I believe in it?" followed by the answer, "I don't' know,..." It is as part of this "I don't know..." answer that "M" and "N" are mentioned.
As it happens, "M" and "N" are metaphorical. These letters represent the millions of Marks and Matthews and Mohamets and the millions of Ninas and Najahs and Nancys who have been wed over the centuries. It also happens that in this soliloquy (of sorts) the Stage Manager/Minister symbolizes an Every Minister sort of character; it is not possible that one minister of the Stage Manager's moderate age (nowhere is the Stage Manager described as being of extraordinarily advanced years) could possibly have married millions of couples.
So "M" and "N" are place holders for unspecified individuals, sort of like the "X" and "Y" in a mathematical equation: There is a number that belongs to each, it is just at the moment symbolically represented and unspecified.