Number density of active and inactive MBHs/MBHBs. Upper panel: number density of MBHs with mass > 10⁸ M_⊙ as a function of redshift, z, for Loud (black) and Boosted model (grey). Middle panel: number density of active (f_Edd > 0.01) and inactive (f_Edd < 0.01) MBHs with mass > 10⁸ M_⊙ as a function of redshift, z, for Loud (black) and Boosted model (grey). Lower panel: number density of MBHBs with chirp mass ℳ > 10⁸ M_⊙ as a function of redshift, z, for Loud (black) model. For reference, it has been shown the results for the Quiet model (grey). In all panels, the error bars correspond to the Poisson error. We remind the reader that the comoving volume provided by the Millennium simulation corresponds to ∼3.6 × 10⁸ Mpc³.