Theme dont have this option but is possible to change inside source code exactly with
/wp-content/themes/businessfinder2/ait-theme/config/cpts-metaboxes/item-data.metabox.php file
Just looks for gallery metabox and there is max value set to 20 by default, which you can change you would like
Please do not submit multiple tickets for the same request, as this does not result in a faster response time.