Hello,

Please try to replace this old code with the following snippet

add_filter(
    'woodmart_product_gallery_settings',
    function() {
        return array(
            'thumbs_slider' => array(
                'items'    => array(
                    'desktop'          => 6,
                    'tablet_landscape' => 3,
                    'tablet'           => 4,
                    'mobile'           => 3,
                    'vertical_items'   => 6,
                ),
            ),
        );
    }
);

Best Regards