(14) And he caught a boy of the people of Succoth, and inquired of him; and he wrote down for him the princes of Succoth, and the elders thereof, seventy and seven men.
Firstly, I want to establish that the people in the time of the Shoftim could write, even the boys. I think this quote quite establishes that.
(ו) וַיֹּאמֶר שָׂרֵי סֻכּוֹת הֲכַף זֶבַח וְצַלְמֻנָּע עַתָּה בְּיָדֶךָ כִּי נִתֵּן לִצְבָאֲךָ לָחֶם.
(ח) וַיַּעַל מִשָּׁם פְּנוּאֵל וַיְדַבֵּר אֲלֵיהֶם כָּזֹאת וַיַּעֲנוּ אוֹתוֹ אַנְשֵׁי פְנוּאֵל כַּאֲשֶׁר עָנוּ אַנְשֵׁי סֻכּוֹת.
We are in the story of Gideon. Gideon's men are hungry, and they are pursuing Zevach and Tzalmunna. They ask the towns of Succot and Penuel for food, and they both refuse.
But what's up with the discrepancy between the singular reply and the plural reply in Succot and Penuel, respectively?
I would like to suggest that Succot was a fortified town, while Penuel just had a central tower. Therefore, the singular in the town of Succot makes sense because they would have sent a letter from inside their fortified town instead of risking opening the gates to talk to Gideon. In Penuel, they had no chance to lock themselves up. This thinking make sense because:
(כה) וַיִּבֶן יָרָבְעָם אֶת שְׁכֶם בְּהַר אֶפְרַיִם וַיֵּשֶׁב בָּהּ וַיֵּצֵא מִשָּׁם וַיִּבֶן אֶת פְּנוּאֵל.
We see that Yeravam had to build Penuel, so obviously it wasn't built up beforehand.