Catholics who believe that no humans will be damned should realize that their belief is, technically, an opinion. That is, it's an intellectual judgment expressing more or less confidence that something is true. We can't now know whether no humans will be damned; we can't have faith that no humans will be damned, since that proposition is not part of the Faith. (We can have faith in someone else's judgment, but that ultimately boils down to an opinion that someone else's opinion is correct.)
The same can be said for any other opinion someone might have about the proportion of the saved. For that matter, introducing an opinion with the words, "I believe," is commonplace whatever the subject, religious or not.
If all these beliefs are actually intellectual judgments, then there's some sort of reasoning that led up to them, reasoning that is more or less sound and that others will find more or less persuasive.
One sort of reasoning that must be ruled out from the start is the sort that depends on or produces a position that contradicts the Faith. Stated baldly, it seems obvious, but sometimes it's hard to derail a train of thought once it starts to roll.
In the context of universal salvation, for example, I think I have often seen reasoning that effectively denies the possibility -- or at least that denied the effective possibility -- of damnation. When someone starts saying things like, "God must reveal Himself irresistibly to everyone," that pretty much makes nonsense of damnation being a "real" possibility.