The fewness of the saved is a long established doctrine of the Catholic Faith. Not defined ex cathedra as far as I know (likely because it was so commonly understood in the past that it never needed to be defined). Alas, we are in need of such definitions now- but the one thing the modernist heresy is very strict on is that nothing anywhere is to ever be clearly defined.
Create billions of souls with free will and original sin. When the vast majority end up in Hell for eternity that's not on God because they all misused their free will.
Then you end up with all the theories about levels of Hell. So, for example, if a Uighur stays true to his faith despite having his family slain and him being tortured to death by the Chicoms he gets to end up in a less hellish part of Hell because he died outside the Church but was true to his heretical beliefs.