Display the number of registrations in a module

I use several forms to let the users register for bus trips, and i would like to put a counter that shows how many people who have signed up for the different trips. Basically a script that shows the last inputs cf_id. Like "There are now (cf_id of latest entry) participants for next weeks trip"

I think this might be useful, but i don't have the know-abouts to make it a script that works in a module:
http://help.joomla.org/content/view/716/60/

[REVIEW] database->insertid
database->insertid

Returns the unique record number of the last record to be inserted into a database table.
Syntax

int insertid ( )
Examples

Example: This example creates a new, default record in the users table and returns the unique id allocated to that user.

// Returns unique id of new user record or false
// if the insert failed.
function newUser() {
  global $database;
 
  $sql = 'INSERT INTO #__users () VALUES()';
  $database->setQuery( $sql );
  if (!$database->query()) {
    echo $database->stderr();
    return false;
  }
 
  return $database->insertid();
}

Can anyone help?