Rashi: Nine for each city, with Ha'Kadosh-Baruch-Hu making up the tenth.

Targum Yonasan: First, Avraham was pleading for five towns to be saved if there were fifty Tzadikim. Since Zo'ar was smaller than the other tows, he asked for it to be saved on the basis of five Tzadikim instead of ten. The regression from fifty down to ten was a matter of how many towns would be saved by the merit of the righteous - four towns if there were forty Tzadikim (and Zo'ar on the basis of HaSh-m's Midas Rachamim); if there were thirty, then three (and Tzevoyim and Zo'ar on the basis of... ), twenty, then two (and Admah, Tzevoyim and Zo'ar on the basis of HaSh-m's Midas Rachamim). And if there were only ten, he concluded, they would join him in prayer to save all five towns.

Seforno: Avraham was asking HaSh-m whether ten in each town is vital in order to save the town, whether it requires an Edah (in which case forty-five will not suffice to save the fifth town),

Ramban: Because He had not yet concluded judgment on Sedom. ¹