When a light ray (I know it’s not a real light ray but it helps to understand ^^) go through a particle, it is reduce by the extinction of this particle (set in the material of the particle).
If the same light ray go through several particles, it is reduce several times by the same amount.
My way to do things : 1 unique big particle. I can control the extinction very precisely ^^
If you want to have a real particle system, you can control his density by controlling the number of particle a light ray have to go through. It’s less precise because you can’t control exactly where the particles are. But yes, in the spawn module, you can choose the average number of particles that an emitter will spawn simultaneously.
This way, you can control (approximatively) the density of your fog. The more you have particles in your system, the more the light ray will go through, the more it will be reduced (I’m not sure it’s still English…) 