Why did Avraham stop at ten? Why did he not ask for nine to save at least one town from extinction?
Rashi: Because a. he saw how, during the Flood, eight Tzadikim were not able to save the world, and b. he had already tried nine - forty-five for the five towns, which is equivalent to nine for one town, and had not succeeded.
Da'as Zekenim, Hadar Zekenim #1: He thought that there were 10 - Lot, his wife, their four daughters, and their four husbands.
Hadar Zekenim #2: Hashem answered that if there are 10 (including Himself) "Lo Ashchis", but He will not exempt without afflictions. If so, eight and Himself will not help. 1
Moshav Zekenim connotes that it might save from death, but in any case there will be afflictions, so he did not request.
Why, in fact, did Avraham not ask for thirty-six, twenty-seven, eighteen and nine - to save four, three, two and one town together with Hakadosh-Baruch-Hu (like he asked for forty-five to save the five towns)?
Rashi (according to Ramban #1): The combination was only acceptable for a large number (five towns), but not for a smaller one. 1
Rashi according to Mizrachi (on Pasuk 29) and Ramban #2: All his future requests involved Hashem combining with the towns concerned. It is obvious that He can similarly combine to make up forty for four towns, thirty for three, twenty for two and ten for one. 2
Targum Yonasan: Refer to 28:1:3.