Genèse 18:32

And he said, I implore mercy before Thee! Let not the anger of the Lord,the Lord of all the world, grow strong, and I will speak only this time. Perhaps ten may be found there; and I and they will pray for mercy upon all the land, and Thou wilt forgive them. And He said, I will not destroy for the sake of the ten who may be innocent.

ויאמר...אך הפעם, Avraham makes clear that if there would not be at least ten righteous people, he would no longer have any argument to present which could be used to help these towns escape their deserved punishment. Significantly, he made no mention of Lot, as he believed that Lot deserved to share the fate of the city having voluntarily associated himself with the wicked inhabitants of that town. Besides, Avraham had no way of knowing if Lot had adopted the perverted lifestyle of the inhabitants of Sodom. He believed that Lot’s only way of saving himself from the doom decreed on the city was to voluntarily depart from that city.

Why did he stop with ten and not continue arguing for fewer? On this matter we find four explanations: A. So that there would be among them a minyan for prayer and numerous other commandments, as it is known that a congregation is ten. B. There were five separate bastions (walled cities), and he intended there would be two (righteous people) in each. C. Experience taught him that there had been eight righteous (Noah, his three sons, and their wives) in the generation of the flood, and they had not pleaded to save that generation on their behalf. D. He reasoned that there were ten (righteous) in Sodom already - Lot and his wife, his four daughters, and their husbands…

אולי ימצאון שם עשרה, “perhaps there can be found there ten?” According to Rashi, Avraham knew already when he had asked if forty five would be sufficient, that G–d would not accept a total of less than fifty in order to save all five towns. He would not accept less than a minimum of ten innocent people per town. If Rashi were correct, why did G–d answer Avraham when he asked about 45 righteous people with the words: לא אשחית, “I will not destroy?” He repeated answering him with the same answer when twenty or thirty righteous people were the subject of Avraham’s question or plea? G–d had made it plain that in order to forgive all five cities there had to be no less that 50 righteous people, i.e. at least ten in each of them. If there were twenty or thirty, the cities containing them would be saved, but none that contained fewer. The promise not to destroy included also not imposing other hardships on those cities at the present state, When Avraham asked about the forty five, he meant that if G–d Himself would not be prepared to be the tenth in each town so that all the inhabitants at this time would be spared. He did not mean that if there were only nine righteous people in each town that they would go scot free; they would not be destroyed but be subjected to a lesser punishment. G–d promised that if there were at least ten righteous people in any one of these cities, not only would He spare that city from destruction, but He would not exact judgments from its guilty people at that time. The number ten is a round number and therefore important enough not only to protect that quorum but also those living alongside them. [Avraham could not have been completely ignorant of this, else why did he not start with a higher number? Ed.] He knew that the eight people in Noach’s ark had not been enough to save the remainder of the human species from destruction at that time, including devastating the earth they had dwelled on. Avraham knew that it would be pointless to ask G–d to desist from destroying all these towns when they could not even point to a quorum of righteous people in any of their cities. At any rate, counting a righteous person in one city, i.e. a city that had eleven righteous people as if the eleven would be added to nine righteous people in another town was also not acceptable. Some commentators hold that the reason that Avraham did not pray for less than ten people was that he thought that Lot, his wife, four children and their spouses, would have made up the quorum. G–d’s response reassured him that if indeed Lot and his family were righteous, they would not be killed in what was about to happen.