Either way you are serving your visitors old data, since W3TC plugin is caching old data and displaying old data. Online visitors also.
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/