how to turn off the number of people online or fake people online

This is fake because your cache plugin cached the forum page and doesn't allow wpForo to update the number dynamically. You should exclude the forum page from your unintuitive cache plugin. Here is the instruction: https://www.redbridgenet.com/how-to-exclude-specific-pages-from-w3-total-cache/

And here is the explanation: https://wpforo.com/community/faq/wpforo-and-cache-plugins/