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<?php 

//Register tag cloud filter callback
add_filter('widget_tag_cloud_args', 'tag_widget_limit');

//Limit number of tags inside widget
function tag_widget_limit($args){

  //Check if taxonomy option inside widget is set to tags
  if(isset($args['taxonomy']) && $args['taxonomy'] == 'post_tag'){
    $args['number'] = 20; //Limit number of tags
  }

  return $args;
}

?>