Close all other programs for more accurate results.
Press the left mouse button to add some amount of gophers.
Adjust the number of gophers that appear at a time with the mouse wheel.
Increase the number of gophers until the FPS starts dropping below 60 to find out your result.
To understand that the drop in performance is not a one-off - use the graphs on the left, they show TPS, FPS and the number of objects over a certain time.
Press the right mouse button to disable batching, this will greatly increase the load, but keep in mind that all measurements were taken without coloring.