The paper investigates the role of mass accretion during Active Galactic Nuclei (AGN) phases in the growth of supermassive black holes (BHs) by comparing the mass function of BHs in the local universe with that expected from AGN relics. The local BH mass function (BHMF) is estimated using correlations between BH mass, bulge luminosity, and stellar velocity dispersion, applied to galaxy luminosity and velocity functions. The density of supermassive black holes in the local universe is estimated to be $\rho_{\text{BH}} = 4.6_{-1.9}^{+1.9} h_{0.7}^{0.2} \times 10^5 \, \text{M}_\odot \, \text{Mpc}^{-3}$. The relic BHMF is derived from the continuity equation, assuming AGN activity is due to mass accretion onto massive BHs and that merging is not significant. The relic BHMF at $z = 0$ is found to be primarily generated at $z < 3$, with smaller BHs growing at lower redshifts compared to more massive ones. The BHMF of AGN relics is consistent with the local BHMF, indicating that local black holes were mainly grown during AGN activity. This agreement is consistent with constraints from the X-ray background, suggesting that merging is not a significant factor in shaping the relic BHMF at low redshifts. The analysis also provides constraints on the accretion efficiency, Eddington ratio, and average lifetime of active BHs.The paper investigates the role of mass accretion during Active Galactic Nuclei (AGN) phases in the growth of supermassive black holes (BHs) by comparing the mass function of BHs in the local universe with that expected from AGN relics. The local BH mass function (BHMF) is estimated using correlations between BH mass, bulge luminosity, and stellar velocity dispersion, applied to galaxy luminosity and velocity functions. The density of supermassive black holes in the local universe is estimated to be $\rho_{\text{BH}} = 4.6_{-1.9}^{+1.9} h_{0.7}^{0.2} \times 10^5 \, \text{M}_\odot \, \text{Mpc}^{-3}$. The relic BHMF is derived from the continuity equation, assuming AGN activity is due to mass accretion onto massive BHs and that merging is not significant. The relic BHMF at $z = 0$ is found to be primarily generated at $z < 3$, with smaller BHs growing at lower redshifts compared to more massive ones. The BHMF of AGN relics is consistent with the local BHMF, indicating that local black holes were mainly grown during AGN activity. This agreement is consistent with constraints from the X-ray background, suggesting that merging is not a significant factor in shaping the relic BHMF at low redshifts. The analysis also provides constraints on the accretion efficiency, Eddington ratio, and average lifetime of active BHs.