This is also a configurable feature. If someone is not seeing it, that’s probably because they didn’t opt into it.
From your user control panel page, select Preferences from the top menu then Interface from the left side menu. Then scroll down to the Other section.
Near the bottom are two items: a checkbox for “Show counts on browser icon” and a dropdown for what to show the count of on the browser tab.
If you do play with it, don’t forget to click the [Save Changes] and refresh the page of interest to see the effect.