I'd like to specify the number of columns which a .card-columns class has by using a class, e.g.
card-columns-{2,3,4}.

This would give the user the opportunity to specify the layout even further. The css would be something like this for the correspoding classes.

.card-columns-2 {
    -webkit-column-count: 2;
    -moz-column-count: 2;
    column-count: 2;
}

Maybe even with the sm, md, lg, xl prefix: card-columns-{sm,md,lg,xl}-{2,3,4}, so you could resolve the issue, that card decks for example aren't responsive currently, but this would lead to responsiveness. I'm currently using the sm prefix for my project to distinct between one and two columns. Works like a charm :)