add_shortcode( 'remaining', 'ff_simple_remaining_places' );
function ff_simple_remaining_places( $atts ) {
    $places_taken = FrmProStatisticsController::stats_shortcode( array('id' => '6', 'type' => 'count') );//change x to the id of the field you want to count
    $remaining_places = 30 - $places_taken; //change 0 to the total number of spaces at the event
    return $remaining_places;
}