This no longer works on Safari >13.0 (Catalina, Mojave, High Sierra) because Apple dropped support for this style of Safari extension.
However some nice folk have implemented the same functionality (and more!) using the new extension framework. You can build it yourself from their github repository or get it for free straight from the Mac App Store.
The old extension still works on earlier versions of Safari. If that's you — read on…
A simple way to stay aware of the number of open tabs in Safari.
By default you get a badge in the toolbar if you have more than sixteen tabs open.
You can click on the toolbar item to see a popup showing how many tabs are open and when the badge will be shown.
You can change when the badge is shown in the Safari Extension Preferences.
