models.physics.particles.particles_per_cell sets how many particles are seeded, as a mean per cell: Volume seeds it times the cells, Surface it times the face cells of the first inlet. It replaces count, which only a PointSource keeps. An injecting inlet takes particles_per_cell in place of number_density, converted with the grid's cells and volume so injected and seeded populations of one value match. Both retired keys are refused with a message naming the replacement.