add_shortcode('EVENT_ATTENDEE_NUMBER','fun_display_attendee_number');
function fun_display_attendee_number($atts)
{
global $wpdb;
$event_id = $atts['event_id'];
$registration_table = $wpdb->prefix.'esp_registration';
$count = $wpdb->get_var("SELECT COUNT(REG_ID) FROM $registration_table WHERE EVT_ID='$event_id'");
return 'Number of registrations: '. $count;
}