bbPress.org

use the bbpress stats widget or shortcode [bbp-stats] in a page

for just all topics it would placing these in a template.

these come the template content-statistics.php which you could copy into child theme into a folder called bbpress and customize.

<?php echo esc_html( $stats['topic_count'] ); ?>

for just replies it would be

<?php echo esc_html( $stats['reply_count'] ); ?>