Contact Form 7 – Make a selection unavailable after a number of bookings

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It is possible with the Smart Grid-layout Extension for CF7C plugin. The smart grid plugin has a [dynamic_dropdown] tag which allows you to filter the options.

dynamic dropdown tag creation screenshot

Replace your dropdown menu with a dynamic dropdown and in the tag Custom Source select the Custom tab, you can then populate the dropdown dynamically using,

add_filter('cf7sg_dynamic_dropdown_custom_options', 'filter_options',10,3);
function filter_options($options, $field_name, $form_key){
  if($form_key != 'my-form') return $options; //check this is the correct form.
  if($field_name != 'bookings') return $options; //check this is the correct field.
  $options = array();
  //load all your booking options.
  foreach($bookings as $booking){
    //verify if the booking limit is crossed.
    if($booking->count < $limit){
      $options[$booking->id] = $booking->title; //$options are $value=>$name pairs.
    }
  }
  return $options;
}