John Gill Commentary Genesis 18:28

"peradventure there shall lack five of the fifty righteous: wilt thou destroy all the city for lack of five? And he said, I will not destroy it, if I find there forty and five." — Genesis 18:28 (ASV)

Perhaps five were lacking of the fifty righteous ,
&c.] Or there should be but forty five; for Abraham perceived, by the Lord's answer, that there were not fifty righteous persons in the place: will you destroy all the city for [lack of] five ?
Abraham proceeds gradually in his requests, and does not ask too much at once, lest he should not succeed: and he said, if I find there forty and five, I will not destroy [it] ;
that is, forty five righteous persons.