We have a statistics widget on the website and app that you can use to view how many flights are being tracked by us at any one time. On the website, you can access the widget from the left-hand side of the screen. If you don't have the widget enabled, you should turn it on from the Widgets section (bottom menu). On the mobile app, you can access the Statistics widget by swiping down from the edge of the top search bar and swiping to the “Stats” tab.
You might also find this Statistics page useful: https://www.flightradar24.com/data/statistics