Some think that this is to be understood as though the saints according to the degree of their merits delivered some thirty, some sixty, some an hundred persons; and this they usually suppose will happen on the day of judgment, not after the judgment. But when this opinion was observed to encourage men in promising themselves impunity, because that by this means all might attain to deliverance, it was answered, that men ought the rather to live well, that each might be found among those who were to intercede for the liberation of others, lest these should be found to be so few that they should soon have exhausted the number allotted to them, and thus there would remain many unrescued from torment, among whom might be found all such as in most vain rashness had promised themselves to reap the fruits of others.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: City of God, book xxi, ch. 27