How to add a result count in exposed filters?
There's no use trying to use hook_form_alter, as you need to wait until the view is rendered.  You can do this in hook_preprocess_views_view__VIEWNAME like this:

function examplemodule_preprocess_views_view__exampleview(&amp;$variables) {
  $result = $variables['header']['result'];
  unset($variables['header']['result']);
  $variables['exposed']['result'] = $result;
}