In this experiment with actionscript 3.0,
I'll be playing around with 100.000 to 300.000 particles at a time.
Create particles is really easy in actionscript, and I'm always doing lots of experiments with lots of sprites or movieClips, and then move then around.
But the problem is that it doesn't take long to have so many particles on the stage that Flash can no longer handle it.
In fact, depending on the code you use for interacting btween this particles, it really doesn't take that many Sprites on the stage at all for Flash to start falling apart.