The Particle Count constraint returns the amount of particles that are located in the vicinity of the currently processed location of the calling node.

This is perfectly suited to restrict the amount of particles in space. In conjunction with a Particle Converter operator (with output set to 'Nirvana') you can delete particles if there are two many. That's what the Particle Grid Emitter does when a fluid dynamics object is assigned.

But you can do a lot more. You can let particle turbulences reduce or increase their intensity depending on the amount of particles.
You can restrict a Surface Reconstructor to exclude particles with too few neighbors (for example to get rid of single particles being included which may speed up the reconstruction process...then you could invert the particle count constraint result in a Krakatoa rendering to only render these particles as small droplets). The list could be extented, it's a great constraint with lots of uses.