When you adjust the extinction parameter in the material of the particle system, you adjust the density (or opacity… The quantity of light that manage to get through the fog).
But if you have 2 particles in the same place, they stop the light twice.
For a better control, I make my particle system with only one immortal particle. Then, I can put 1 big particle every where I want some fog.
If you want an animated fog (with multiple particles, then), you should use a very small extinction and set the density of your fog by altering the quantity of particle spawned. The only problem I see with this is that you can’t control how many particles each players will have because they can reduce the number of particles through the scalability settings.
That’s why I choose to put each particle manually in my world 