Well…given you provided the link he gives a pretty good description on how to achieve the result.
These are not particles but instanced low poly blades of grass. You can do this through the same exact method he used and described.
The maximum number of instances is going to be limited based on the machine the application is running on and the browser space available.
Depending on how far away the camera is from the grass, a simple sprite based animation may work as well. If you need close up blades that have to be in 3D maybe a hybrid of the two. UV cycling is far more efficient than 1000s of objects but if your scene calls for only 3D then…you will have to have determine how many you can get away with.