I'm not sure its The Prisoner. I emailed her the suggestion. I saw the tv show years ago.
She said 'science fiction story'. So I don't think Letter People is it.
She emailed more info.
She read the story in a university class 'Survey of Contemporary Literature. The book was about characters who were actually numbers in a science fiction setting... (snip) the "character names" looked something like the title of a similar book: Ms Fnd in a Lbry (1961)... (snip) mathematics novel of science fiction. I remember also the setting was outer space if I remember correctly.'