How to limit the number of comment a user can post?
You could add a validate handler to the comment form

e.g, something like:

/**
 * hook_form_FORM_ID_alter
 */
function MODULE_form_comment_form_alter(&amp;$form, &amp;$form_state, &amp;$form_id) {
  $form['#validate'][] = 'MODULE_comment_form_validate';
}

/**
 * Validate comment form submission
 */
function MODULE_comment_form_validate($form, &amp;$form_state){
  $node = node_load($form_state['values']['nid']);
  if(isset($node-&gt;type) &amp;&amp; $node-&gt;type = 'targetted_type'){
    $account = user_load_by_name($form_state['values']['name']);
    if(!empty($account-&gt;uid)){
      // Get comment count for this node type
      $query = db_select('comment', 'c');
      $query-&gt;join('node', 'n', 'n.nid = c.nid');
      $query-&gt;condition('c.uid', $account-&gt;uid);
      $query-&gt;condition('n.type', 'targetted_type');
      $query-&gt;countQuery();
      $comments = $query-&gt;execute()-&gt;fetchField();
      if($comments &gt; variable_get('max_comments_targetted_type', 3)){
        form_set_error('name', 
          t('You are only allowed to post %max comments on this type of content.', array('%max' =&gt; variable_get('max_comments_targetted_type', 3))));
      }
    }
  }
}