谢谢Pieter。

基于this answer, 我对特定类别的工作做了一些更改。这是我的最终代码：

<div class="lessonNumber">
      <?php 

        class MY_Post_Numbers {

            private $count = 0;
            private $posts = array();

            public function display_count() {
                $this->init(); // prevent unnecessary queries
                $id = get_the_ID();
                echo __(\'שיעור\', \'swgeula\') . \' \' . $this->posts[$id] . \' \' . __(\'מתוך\', \'swgeula\') . \' \' . $this->count;
            }

            private function init() {
                if ( $this->count )
                return;
                $parent_cat = get_the_category()[0];
                $parent_cat_id = $parent_cat->cat_ID;
                global $wpdb;  
                $posts = $wpdb->get_col( "SELECT ID FROM $wpdb->posts WHERE post_status = \'publish\' AND post_type = \'post\' AND ID IN ( SELECT object_id FROM {$wpdb->term_relationships} WHERE term_taxonomy_id = \'"  . $parent_cat_id . "\' ) ORDER BY post_date " ); 
       
                // can add or change order if you want 
                $this->count = count($posts);
  

                foreach ( $posts as $key => $value ) {
                    $this->posts[$value] = $key + 1;
                }
                unset($posts);
            }

        }

        $GLOBALS[\'my_post_numbers\'] = new MY_Post_Numbers;

        function my_post_number() {
            $GLOBALS[\'my_post_numbers\']->display_count();
        }

        my_post_number(); 

    ?>
</div>