... hide the main image and only show thumbnails, and then when a thumbnail is clicked have the main image come up and hide the thumbnails?
This is certainly possible. You can force the gallery to always be displayed in Small Screen Mode (where thumbnails and main images are displayed on different pages) by setting screenMode="SMALL" (in JuiceboxBuilder-Pro's 'Customize -> General' section).
More information about Screen Modes can be found here.
Here is a sample gallery which demonstrates Small Screen Mode: https://www.juicebox.net/demos/pro/ssm/
I would probably like to have 20 or so thumbnails taking up the whole screen in 5 rows,
The thumbnail pages in Small Screen Mode are responsive and it is not possible to set the exact number of thumbnails (columns and rows) per page. The number of thumbnails displayed on a thumbnail page in Small Screen Mode will be determined by the thumbnail dimensions (set via the thumbWidth and thumbHeight configuration options) and the size of the user's browser window. If there is not enough available space to display all the thumbnails at once, then additional thumbnail pages will be generated and the user can navigate between them with the thumbnail paging arrows.
Take a look at this sample gallery and try resizing your browser window: https://www.juicebox.net/demos/pro/full … Mode=SMALL
... and then when a thumbnail is clicked and the main image opens the thumbnails revert to 1 row of four (scrollable) at the bottom.
This is not possible. In Small Screen Mode, the thumbnails and main images are always displayed on different pages (never together) and in Large Screen Mode, the thumbnails and main images are always displayed together on the same page (although the thumbnails can be toggled on and off via the Thumbnail Button or with the auto-hide functionality).
Here's a sample gallery featuring the Thumbnail Button (on the gallery's Button Bar at the top right) which shows and hides the thumbnails when clicked: https://www.juicebox.net/demos/pro/default/