Genesis 18:24

Perhaps there are fifty innocent persons within the city, who pray before Thee,--ten for every city, of all the five cities of Sedom, Amorah, Admah, Zeboim, and Zoar. Wilt Thou in Thy displeasure destroy and not forgive the country, on account of the fifty innocent ones who are in it? Unholy would it be before Thee to do according to this word, to slay the innocent with the guilty, and to make the innocent to be as the guilty!

FIFTY RIGHTEOUS. Rashi wrote: “Ten righteous men for each city. Wilt thou destroy on account of the five. Nine for each city, and You, the All-Righteous One of the Universe, will be counted with them [to make up the original number of ten]. Perhaps there shall be found there forty. Then let four cities be saved. So, too, thirty will save three of them, twenty will save two of them, and ten will save one of them. And he did not plead for less than ten since in the generation of the flood there were eight righteous people, and they could not save their generation. For nine, in association with G-d, he had already pleaded but found no acceptance.” All these are the words of the Rabbi, of blessed memory.
But I wonder: If so, what is this prayer and supplication which he pleads each and every time, saying, oh let not the Lord be angry; Behold, now, I have taken upon me to speak? It is proper that forty should save four cities, and thirty and twenty should save in proportion, just as fifty would save five! Similarly, concerning that which Rashi said, “for nine in association with G-d he had already pleaded but found no acceptance,” it may be asked: When he pleaded about forty-five, [i.e., to save all five cities by having nine righteous men for each city] in association [with G-d to make up ten], and he did not find forty-five, but perhaps he might have found there nine! Now it would seem that the intention of the Rabbi is that many righteous people can effect a proportionately greater salvation than a few righteous people can, just as the Sages have said:) “A few who fulfill the commandments of the Torah cannot compare with the many who fulfill the commandments of the Torah.” And thus, the Holy One, blessed be He, having conceded that forty-five righteous men in association with the All-Righteous One of the Universe would save all the five cities just as if there were the entire fifty, it follows that if forty could save four cities — in association with the Righteous One, praised be He — they would also save with even thirty and twenty, since He already conceded this association. [Thus, thirty-six would save four, twenty-seven three, eighteen two, and nine one]. And in case you say that He conceded only the case of forty-five because they are many, and perhaps He might not concede the principle of association with the few, as we have said, the refutation is that it is proper for the righteousness of G-d to associate even with the few and save [as many of the cities as possible] since He had conceded the principle of association, for He would not distinguish between the many and the few. This is the opinion of the Rabbi.
But the way of the simple meaning of the verses is smooth. First Abraham said fifty in order to give a perfect number of ten for each city, and then he decreased the number as much as possible, and each time he thought to save all five cities. And I do not know who brought the Rabbi to that which he said.

בתוך העיר, Sodom. The king of Sodom was the senior monarch of all the satellite towns surrounding Sodom.

?האף תספה ולא תשא למקום, even though You are justified in destroying the wicked on account of the righteous, far be it from You to kill the righteous.

?אולי יש חמשים צדיקים בתוך העיר, the words בתוך העיר must be understood as referring exclusively to the city of Sodom, not including its satellites. We know this from G’d’s reply in which He spelled out: in verse 24 “if I find in Sodom 50 righteous men inside the city, etc.” Avraham’s argument could have been that if there would be 50 righteous men in the city of Sodom, it was most likely that also the satellite towns would harbour among their populations some righteous people. Alternately, Avraham may have asked that if there would be a total of 50 good men scattered in the various towns, this should be enough to allow all the cities to escape destruction at this time. G’d might then have mentioned Sodom as an example of all of the cities in the valley, saying that 50 people would suffice for G’d’s forgiveness at this time. He singled out Sodom because Lot, Avraham’s kin, lived in that particular city. Avraham tested the number of righteous men that would be the rock bottom minimum for G’d to suspend all or part of the destruction He had planned.