Or HaChaim on Genesis 18:28/אור החיים על בראשית י״ח:כ״ח

1 אולי יחסרון, Perhaps there will be five short?" Abraham implied that inasmuch as G'd was righteous He would make up the tenth in each of the five towns which had only nine righteous men in their own right. The assumption underlying all this is that at least a quorum of ten is needed in each town for those ten good men to provide protection for their town.