// in my AppController

public function beforeFilter() {

        $this->set('totalSubmissions', $this->getSubmissionCount());
}

    public function getSubmissionCount() {
        $totalSubmissions = $this->Submission->find('count');
        return $totalSubmissions;
    }

// in my app/views/elements/sidebar_categories.ctp (which is called in app/views/default.ctp)

<?= $totalSubmissions; ?>