פּירוש אויף בראשית 42:13

'ויאמרו שנים עשר עבדיך וגו AND THEY SAID, THY SERVANTS WERE TWELVE BRETHREN — [AND ONE IS NO MORE], and it is on account of that one who is no more that we dispersed ourselves through the city in order that we might search for him (Genesis Rabbah 91:6).

ויאמרו...בני איש אחד, they now felt the need to assert further that they were in fact honest and forthcoming by revealing that actually there were twelve of them, all sons of the same father, etc.

שנים עשר עבדיך אחים, בני איש אחד בארץ כנען, what we have said before can easily be proven. After all, our father is still alive in the land of Canaan. Both he and his neighbours will confirm what we have told you, i.e. that we used to be twelve brothers, that one went missing, and that the youngest remained to look after the affairs of our father’s household. You can confirm that we have spoken the truth by checking out all these details.