How to get post_id when new or edit posts?

I am trying to make a unique ACF field using this
https://support.advancedcustomfields.com/forums/topic/accept-only-unique-values/

add_filter('acf/validate_value/name=numero', 'unique_numero_field', 10, 4);

function unique_numero_field($valid, $value, $field, $input) {
  return $_POST['post_id'];
}

How to get the post_id here?
When setting the dynamic values for title as post_id – the post id is showing in the form title.