How can I add more columns to the calendar's Resource Scheduling screen?
All you need to do is change the number of repetitions shown for each of the repeating fields on this layout. There are a few stacked fields here so take this layout apart in a practice copy before you try this for real. The movie linked to here should help you get familiar with these stacked objects.
Note that under the hood our calendar supports 20 columns on this screen without any modification to the underlying code. If you want more than 20 columns to show, you'll need to change 3 things:
Of course, adding columns can slow the calendar down as there is more to draw, more work to do, more columns to consider, so see if you can live within the current schema before adding columns. Remember that you can filter the resources which show up, so you only see the ones relevant to you.