How do I call a controller function in a module?
If you want to define a controller, then don't use a static method:

namespace Drupal\content_statistics\Controller;

use Drupal\Core\Controller\ControllerBase;

class ContentStatisticsController extends ControllerBase {

  public function count() {
    $value = $this-&gt;t('hi');
    return $value;
  }

}


The use statement should import the class, not the namespace. But this is not the only problem.

If you want to use a controller class in a hook, create an instance of the controller and use the method of that instance:

use Drupal\content_statistics\Controller\ContentStatisticsController

$statistics = new ContentStatisticsController;
$value = $statistics-&gt;count();


Your class and method had the same name, I split the name in a controller class name and the method count().