Contact Form submit more than 5 times in Drupal 8

Increase flood limit and flood interval in Drupal 8

Write down function in theme file

function theme_preprocess_page(&$variables) {
    $flood_limit = 500; //Default value is 5, change to whatever suits you.
        \Drupal::configFactory()->getEditable('contact.settings')
        ->set('flood.limit', $flood_limit)
        ->save();
}

There are two option below-:

//Default value is 5, change to whatever suits you. 

$flood_limit = 500;
        \Drupal::configFactory()->getEditable('contact.settings')
        ->set('flood.limit', $flood_limit)
        ->save();



//Default value is 3600sec (1hour), change to whatever suits you

$flood_interval = 3600;
 \Drupal::configFactory()->getEditable('contact.settings')
      ->set('flood.interval', $flood_interval)
      ->save();