If you use an exposed filter and want to show the unfiltered number of total results (like "we are showing you 12 items out of total 345 items"), this gets slightly more complicated. In a preprocess_views_view
or any other preprocess which gets the view as a variable, run
// Pass original view row count to template
$pager = \Drupal::service('pager_manager');
$pager = $pager_manager->getPager();
$request_stack = \Drupal::requestStack();
$request = clone ($request_stack->getCurrentRequest());
$request->query = new ParameterBag;
$request_stack->push($request);
$original_view = $variables['view'];
$view = Views::getView($original_view->id());
$view->setDisplay($original_view->current_display);
$view->preExecute($original_view->args);
$view->execute();
$view->postExecute();
$request_stack->pop();
$pager_manager->createPager($pager->getTotalItems(), $pager->getLimit());
$variables['total_rows'] = $view->total_rows;