Based on classical mean-field approximation using the diffusion equation for ergodic normal motion of single 24-nm and 100-nm nanospheres, we simulated and measured molecule number counting in fluorescence fluctuation microscopy. The 3D-measurement set included a single molecule, or an ensemble average of single molecules, an observation volume DeltaV and a local environment, e.g. aqueous solution. For the molecule number N << 1 per DeltaV, there was only one molecule at a time inside DeltaV or no molecule. The mean rate k of re-entries defined by k = N/tau(dif) was independent of the geometry of DeltaV but depended on the size of DeltaV and the diffusive properties tau(dif). The length distribution l of single-molecule trajectories inside DeltaV and the measured photon count rates I obeyed power laws with anomalous exponent kappa =-1.32 approximately -4/3.