Rashi to Bereishis 18:32, explaining why Avraham only bargained for 10 and no less:
אָמַר דוֹר הַמַּבּוּל הָיוּ ח', נֹחַ וּבָנָיו וּנְשֵׁיהֶם, וְלֹא הִצִּילוּ עַל דּוֹרָם; וְעַל ט' עַל יְדֵי צֵרוּף כְּבָר בִּקֵשׁ וְלֹא מָצָא
He said, "The generation of the Flood had eight [righteous people] - Noach, his [three] sons, and their [altogether four] wives - and they were unable to save their generation." He did not ask for nine plus Hashem, because he already asked that [when he asked for 45 to save the area].
Why is this true? Maybe eight (nine) have sufficient merit to save a city, but not the entire world. If the argument went the other way, I could hear that, but the way it's written, it seems backwards.
This related question asks why Lot was successful with just four, while Avraham wasn't able to argue that. While it's a similar question, what I'm asking is that Avraham's own logic doesn't seem to make sense - the reason he has not to plead further seemingly isn't a valid argument.