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): The combination was only acceptable for a large number (five towns), but not for a smaller one. 1
Rashi according to Mizrachi (verse 29), Ramban: All his future requests were with Hashem joining Himself. It is obvious that He can similarly join to make forty for four towns, thirty for three, twenty for two and ten for one. 2
רש"י: ועל תשעה על ידי צירוף כבר בקש ולא מצא: אם הכוונה שביקש כבר על ארבעים וחמשה (תשעה לכל עיר), קשה שאולי תשעה אכן ישנם?
גור אריה: כוונת רש"י שאברהם כבר ביקש להחשיב תשעה צדיקים ע"י צירוף, וא"כ כשביקש על שלושים ועשרים וכו' הייתה כוונתו לתשעה וע"י צירוף.