function wt_get_category_count($input = '') {
 global $wpdb;

 if($input == '') {
  $category = get_the_category();
  return $category[0]->category_count;
 }
 elseif(is_numeric($input)) {
  $SQL = "SELECT $wpdb->term_taxonomy.count FROM $wpdb->terms, $wpdb->term_taxonomy WHERE $wpdb->terms.term_id=$wpdb->term_taxonomy.term_id AND $wpdb->term_taxonomy.term_id=$input";
  return $wpdb->get_var($SQL);
 }
 else {
  $SQL = "SELECT $wpdb->term_taxonomy.count FROM $wpdb->terms, $wpdb->term_taxonomy WHERE $wpdb->terms.term_id=$wpdb->term_taxonomy.term_id AND $wpdb->terms.slug='$input'";
  return $wpdb->get_var($SQL);
 }
}

<?php echo wt_get_category_count(); ?>

<?php echo wt_get_category_count(1); ?>

<?php echo wt_get_category_count('hello-world'); ?>

<?php echo strip_tags(wp_list_categories('include=3&hide_empty=0&use_desc_for_title =0&echo=0&show_count=1&style=none&hierarchical =0&title_li=')); ?>

<?php
 // 将以下category-name改成你的分类别名即可
 echo get_category_by_slug('category-name')->count; 
?>

<?php
 // 将以下cat_ID改成你的分类ID即可
 echo get_category(cat_ID)->count; 
?>

function ludou_get_cat_postcount($id) {
 // 获取当前分类信息
 $cat = get_category($id);

 // 当前分类文章数
 $count = (int) $cat->count;

 // 获取当前分类所有子孙分类
 $tax_terms = get_terms('category', array('child_of' => $id));

 foreach ($tax_terms as $tax_term) {
  // 子孙分类文章数累加
  $count +=$tax_term->count;
 }
 return $count;
}

<?php
 echo 'ID为123的分类及其子孙分类的文章数量为：' . ludou_get_cat_postcount(123);
?>