A series of coupled direct numerical simulations of decaying Homogeneous Isotropic Turbulence are performed on a 128 3 perodic box, with a size monodisperse population of particles at different Stokes numbers St, mass loading φ and particle number densities n 0 (mean number of particles per unit volume). Indeed, mechanisms for turbulence enhancement or suppression depend on these three combined parameters. Like previous investigators, crossover and modulations of the fluid energy spectra were observed consistently with the change in Stokes number and mass loading. Additionally, we also investigate the impact of the particle number density, which was not properly and independently characterized. This parameter plays a key role in the statistical convergence of the disperse phase and is therefore of primary importance for modeling purposes. DNS results show a clear impact of this parameter, promoting the energy at small scales while reducing the energy at large scales. Our investigation focuses on the identification of the energy transfer mechanisms, to highlight the differences between the influence of a homogeneous disperse phase (very high particle number density where statistical convergence is obtained) and heterogeneous cases (low particle number density). In particular, different regimes have been identified and described in terms of particle number density.