Sadly due to snow and a lack of a pc nearby, I can't test the following. You could store the particle number in script and then when more particles exist try something like:
See http://unity3d.com/support/documentation/ScriptReference/Particle.html for more info. See http://unity3d.com/support/documentation/ScriptReference/ParticleEmitter-particles.html to learn how to get and set particles from the emitter.
I hope this helps :)