Which hook to implement to limit node creation? I'm looking to limit the number of nodes that a user can create in Drupal 7.  

Which hook should I implement to step in before the 'add node' page is shown?
Alter the access callback for the menu callback:

function custom_module_menu_alter(&amp;$items){
    $items['node/add/page']['access callback'] = 'my_custom_access_callback';
    // Next line needed for Drupal 7
    unset($items['node/add/page']['access arguments']);
}


Add a function as your custom access callback:

function my_custom_access_callback() {
   global $user;
   $allowed_limit = variable_get('allowed_limit', 10);

   // If Drupal 6.
   $node_counts = db_result(db_query("SELECT count(*) FROM {node} WHERE uid = %d;", $user-&gt;uid));
   // End If

   // If Drupal 7.
   $node_counts = db_query("SELECT count(*) FROM {node} WHERE uid = :uid", array(':uid' =&gt; $user-&gt;uid))-&gt;fetchField();
   // End If

   if ($node_counts &lt; $allowed_limit &amp;&amp; user_access("create page")) {
     return TRUE;
   }
   else{
     drupal_set_message(t('Print message about exceeded limit and/or user permissions.'));
     return FALSE;
   }
}


This kind of logic will restrict a user to exceed the limit.