How to put limit on slider

I installed the Lite version of this theme on a test site and dug around. The function that generates the slider doesn’t provide any useful filters. However, it does store the IDs of the featured posts in a transient, so you could use the pre_set_site_transient_' . $transient and transient_' . $transient filters to reduce the number of posts. It also respects the value ‘max_posts’ as declared in the theme, so you could replace this. Both would required creating a child theme (or, in the case of the former, a plugin that allows you to add hooks would also suffice).

Transient method:

add_filter( 'pre_set_site_transient_featured_content_ids', 'wpse174427_transient_method' );
add_filter( 'transient_featured_content_ids', 'wpse174427_transient_method' );
function wpse174427_transient_method( $value ) {
  return array_slice( $value, 0, 5 );
}

Theme support method:

add_action( 'after_setup_theme', 'wpse174427_themesupport_method', 11 );
function wpse174427_themesupport_method() {
  remove_theme_support( 'featured-content' );
  add_theme_support( 'featured-content', array( 'featured_content_filter' => 'dynamicnews_get_featured_content', 'max_posts' => 5 ) );
}