How to know how many elements are processed in the finish callback?
Reading the Batch Process in Drupal 8 and Drupal 8 Batch Example articles I found the answer.

The key is in the 

//Message to show during each execution of the callback function
$context['message']
//Array with the total of processed items.
$context['results']


So, the solution is in the operations callback function, now the functions is like this and it works:

function process_line_saint_du_jour($line, &amp;$context) {
  //Custom Code
  $info = str_getcsv($line, ";", '"');
  $context['results'][] = $info[3];
  $context['message'] = t('Migrating Saint du jour @day', array('@day' =&gt; $info[3]));
}