Space-time plots showing the solid surface density for runs with q = 3.5, τ_s = 10⁻³−10⁻¹ and N_p = 7, 70, 140, 280 vs. 700 per cell. The last panel, indicated with a dagger, corresponds to the run where N_p = 700 per cell and the particles are initialized with a different randomization as in our nominal 700 per cell model, shown on the left-hand side of the last row. The orange line marks the limit of threshold metallicity, Z_crit, where ρ_p,max∕ρ_g = 100. The y-axis on the right-hand side shows the metallicity, Z. In all four systems, the critical metallicity is reached after about 100–120 orbits, signifying that increasing the particle number does not make the multispecies streaming instability less efficient in forming filaments.