Hmm, ok, so that means we have an n electron system $\bigotimes_{i=1}^{N_e}\Bbb{R}^3$ and we get the density matrix of this system. The one electron RDM is then tracing out all except one electron
$$\gamma (\vec{x},\vec{x}')=\int \Psi^*(\vec{x}',\vex{x}_2,\dots ,\vec{x}_{N_e}) \Psi^*(\vec{x}',\vex{x}_2,\dots ,\vec{x}_{N_e})$$