Counting number of flagged nodes

[I've deleted my comment: I'll return when I have more time and expand it.]

(1) You're using Flag 1.x. The hooks described in the document you linked to exist in Flag 2.x only. So they won't work for you.

(2) HOWEVER, you're going to be disappointed even after upgrading to Flag 2.x, because, as that document points out, using those hooks you can't show an error message to the user. Your users won't like it. It's one thing #952114: Have hook_flag_validate() will solve.

(3) An alternative solution: Implement hook_flag() and when you see that an item has been flagged, unflag it if more than 2 items (in your case) are already flagged. It ought to work in Flag 1.x as well. Here's code to get you started (use it as a starting point: I may have typos):

/**
 * Implements hook_flag().
 */
function MYMODULE_flag($action, $flag, $content_id, $account) {
  if ($flag->name == 'borrow' && $action == 'flag') {
    if ($flag->get_user_count($account->uid) > 2) {
      $flag->flag('unflag', $content_id, $account);
      $flag->flag_message = $flag->unflag_message = "You can only borrow 2 items at a time. Please unborrow some item first.";
    }
  }
}

(The document you used contains a complicated code to count the items but that's only because it counts over a span of time. In your case, since you want the total count, one method call is all that's needed.)

(One disadvantage of this snippet is that if some actions are triggered in the 'flag' event (e.g., unpublishing a node), they'll have to be undone. hook_flag_validate() won't have this problem.)

I added these clarifications to the document(s) you linked to.

An HTTP error 200 occurred.

This means a PHP error occured. Something inside the if() {} is wrong.

You're clicking the link with your left mouse button. So you can't see the errors. Open the link in a new window. Or disable JavaScript. And if you see a white screen, check your server's error log.

Maybe you want to check at your end as well

I don't have the time for this. I already said my code is only a starting point. Let us know what errors are printed (the _real_ error(s), not the one you quoted) and we'll try to help.

I've checked the code I gave you and it works for me. So I'm marking this "fixed".

Re-open this when you have the error message.

IMPORTANT: The code has "$flag->get_user_count($account->uid)". This will work in Flag 1.x only. If you're using Flag 2.x, change this to "$flag->get_user_count($account->uid, flag_get_sid($account->uid))". (This won't be necessary when #1011048: $flag->get_user_count(): make 'sid' optional is fixed.)

After searching for a way to create a simple voting system in which each user would only have 5 votes each, this code worked perfect. I wanted the simplicity of Flags to vote for items and did not find what I needed from any of the voting modules. This helped my tremendously. Thanks for the discussion and for sharing this code.