For information, the only thing I can do to solve my issue is to modify and remove line 972
// else if($n>10000) $number = round(($n/1000),1).' '.str_replace('{number}', '', wpforo_phrase('{number}K',false));
in ../wp-content/plugins/wpforo/wpf-includes/functions.php
But at the first update, i will have to remove the line again
Just remove all changes and read the answer for the topic opener. The issue is located in the PO file for your website language: https://wpforo.com/community/how-to-and-troubleshooting-2/issue-with-posts-count/#post-32737
Even if you've not made any changes, it comes from plugin /languages/ folder. Install Loco Translate plugin and edit your language file of wpForo, find the phrase and change it to the English version.
Don't forget delete phrase cache in Dashboard > Forums > Dashboard admin page.
Thanks for your help
I finally succeded with installing loco translate and modify french language