$node->processed['add_collection_form'] = drupal_get_form('add_to_collection_form', $node->nid);

print drupal_render($node->processed['add_collection_form']);

function hook_forms($form_id, $args)
{
  $forms = array();

  // Check if the form id is of the pattern add_to_collection_form-[NUMBER]
  if(preg_match('/^add_to_collection_form-d+$/', $form_id)
  {
    $forms[$form_id]['callback'] = 'add_to_collection_form';
  }

  return $forms;
}

drupal_get_form('add_to_collection_form-1', $node->nid);
drupal_get_form('add_to_collection_form-2', $node->nid);
drupal_get_form('add_to_collection_form-3', $node->nid);