Another interpretation. Why ten? Because [Abraham] reasoned: surely there in Sodom are Lot and his wife and his four daughters and his four sons-in-law [making ten, enough to save the city]. Rabbi Yehudah ben Rabbi Simon and Rabbi Chanin in the name of Rabbi Yochanan said: here [in Sodom it took] ten, but in Jerusalem even one [would suffice], as it is written, "Roam through the streets of Jerusalem... [and see now, and know, and seek in her open places, if you can find a man, if there be any that does justice, that seeks truth, and I will pardon her]" (Jeremiah 5:1).
Abraham Counted on Lot's Household to Reach Ten
Yalkut Shimoni on Torah 83:12
דָּבָר אַחֵר לָמָּה עֲשָׂרָה, שֶׁהָיָה סָבוּר הֲרֵי שָׁם לוֹט וְאִשְׁתּוֹ וְאַרְבַּע בְּנוֹתָיו וְאַרְבָּעָה חֲתָנָיו. רַבִּי יְהוּדָה בֶּן רַבִּי סִימוֹן וְרַבִּי חַנִּין בְּשֵׁם רַבִּי יוֹחָנָן, כָּאן עֲשָׂרָה וּבִירוּשָׁלַיִם אֲפִלּוּ אֶחָד, דִּכְתִיב "שׁוֹטְטוּ בְּחוּצוֹת יְרוּשָׁלַיִם" וְגוֹ'.