How do I modify a Drupal Theme?
If I understand your (main) question correctly, you want to show additional images in the slider. In that case you have to narrow down which modules are used to create the slider. Typically a View is used to retrieve all content (stored in your database, normally not encrypted), an overview of all Views can be found at admin/structure/views. In the View there is an option to set the number of items that have to be shown in the Pager section (setting the number of items to show to 12 would in a typical installation be the thing you want to do).

When this is indeed the set up and you don't see the expected change after altering the value, don't forget to clear the caches (drush cr or via admin/config/development/performance).

If your environment is different, please provide additional details: which module is used to create the slider, e.g. Flex Slider, is Views used to provide the data (images provided by the user), etc.