  We present a method to compute the number of particles occupying spherical
single-particle (SSP) levels within the energy density functional (EDF)
framework. These SSP levels are defined for each nucleus by performing
self-consistent mean-field calculations. The nuclear many-body states, in which
the occupation numbers are evaluated, are obtained with a symmetry conserving
configuration mixing (SCCM) method based on the Gogny EDF. The method allows a
closer comparison between EDF and shell model with configuration mixing in
large valence spaces (SM-CI) results, and can serve as a guidance to define
physically sound valence spaces for SM-CI calculations. As a first application
of the method, we analyze the onset of deformation in neutron-rich $N=40$
isotones and the role of the SSP levels around this harmonic oscillator magic
number, with particular emphasis in the structure of $^{64}$Cr.
