Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?board=21.0]Examples for beginner who want to learn EJS[/url]

You will find [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=2407.0]Video show you how to create simulation with EJS[/url] and more examples.

The # of particles can be simulated is limited by your memory and CPU speed.
Normally, it is not difficult to simulate  hundred of particles.