Wordpress - 如何在预览the_excerpt中只显示两个句子

I need my preview show only two sentences, how can I do it?

    add_filter(
    'the_excerpt',
    function ($excerpt) {
      return substr($excerpt,0,strpos($excerpt,'.')+1);
    }
  );

  function excerpt_read_more_link($output) {
      global $post;
      $output = preg_replace('/(.*?[?!.](?=\s|$)).*/', '\\0', $output);
      if(mb_strlen($output) > 300) {
          $output = mb_substr($output, 0, 300);
        }
        return $output; 
    }
    add_filter('the_excerpt', 'excerpt_read_more_link');

I have a code, that make it for first one sentence, but how can I fix it to show two?