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function dj_modify_num_posts_for_testimonials($query)
{
    if ($query->is_main_query() && $query->is_post_type_archive('testimonials') && !is_admin())
        $query->set('posts_per_page', 20);
}
 
add_action('pre_get_posts', 'dj_modify_num_posts_for_testimonials');