It is interesting that Darnay uses the number fifty-two for those prisoners of the Conciergerie who must go to their deaths in Chapter XIII of Book the Third. For, in his meeting with John Barsad, who is really Solomon Pross, Darnay employs the metaphor of a game of cards--which has fifty-two in the deck--being laid down, or played, in his argument that Basard must do as he proposes:
"I hold another card, Mr. Barsad. Impossible, here in raging Paris, with Suspicion filling the air, for you to outlive denunciation, when you are in communication with another aristocratic spy...who, moreover, has the mystery about him of having feigned death and come to life again! [Roger Cly] A plot in the prisons, of the foreigner against the Republic. A strong card--a certain Guillotine card! Do you play?"
Carton, thus, threatens to expose Barsad's past spying as well as his connection to Roger Cly unless he assist Carton in his plan. So, although Barsad says he "throw[s] up" his cards, he must comply or else with Carton's denunciation he will die. Carton's plan, spawned from his love for Lucie and his desire to redeem his dissipated life, is to enter the prison and trade places with Darnay, saving him for Lucie Manette and their daughter.
In Chapter XIII, "Fifty-Two," then, all the figurative cards having been played, he and the other doomed prisoners of the"black prison" of the Conciergerie, who, significantly, number fifty-two, are brought in tumbrils to the guillotine for execution.