Notice: Undefined offset: 166
It sounds like you have reached your current PHP max_input_vars limit.
Each image in a Showkase gallery page has several different variables associated with it and, in order to process a large gallery, you will need to ensure that max_input_vars is large enough to accommodate all the images. (It should be set to at least 7 x the number of images in your largest gallery.)
It sounds like your max_input_vars might currently be set to 1000 (as the undefined offsets start at image #166).
Try increasing your max_input_vars, perhaps to 2000 (or even larger to give yourself a buffer for very large galleries).
You should be able to check your current PHP settings in your web hosting Control Panel or by using phpinfo() as follows:
(1) Create a new file in a plain text editor with the following code: <?php phpinfo(); ?>
(2) Save the file with a .php file extension (for example, 'phpinfo.php').
(3) Upload the file to your web server and open it in a browser.
(4) Search the web page for max_input_vars.
You should be able to change PHP values using one of the following methods:
(1) Via a php.ini file.
(2) Via an .htaccess file.
(3) Via your web hosting account's online Control Panel.
If you have trouble changing your PHP max_input_vars value, your web host should be able to help you out.
Only one thumbnail displays and not rows and columns.
From your screenshot, it looks like this is due to your thumbnail dimensions (and the available screen size).
It looks like there is not enough room within the mobile device's browser to display more than one thumbnail.
The only solution to this (to allow more thumbnails to be displayed on the mobile device pictured) would be to reduce the thumbWidth and thumbHeight (in the 'Thumbnails (Pro)' section of your gallery page).
I notice you have increased both the thumbWidth and thumbHeight to 150 (from the default values of 85).
More thumbnails will be displayed per page if you revert to the default values of 85.
Unfortunately, it is not possible to use different thumbnail dimensions for Large Screen Mode and Small Screen Mode so a compromise may need to be made, especially if the majority of your target audience is likely to be viewing your galleries on small screen devices.
Can I edit the juicebox gallery so that I can have at least 3x5 rows of images?
You cannot specify the number of thumbnails to be displayed on each thumbnail page in Small Screen Mode.
The number of thumbnails displayed per page is determined by the thumbnail dimensions and the space available in the browser viewport.
If you load your gallery's thumbnail page in a desktop browser and then change the size of the browser window, you'll see the number of thumbnails dynamically change (and the number of thumbnail pages will also dynamically change).
I hope my notes above help.
Please let me know how you get on and if I can be of any further assistance.