  We explore the mass-to-light ratio in galaxy clusters and its relation to the
cluster mass. We study the relations among the optical luminosity ($L_{op}$),
the cluster mass ($M_{200}$) and the number of cluster galaxies within
$r_{200}$ ($N_{gal}$) in a sample of 217 galaxy clusters with confirmed 3D
overdensity. We correct for projection effects, by determining the galaxy
surface number density profile in our cluster sample. This is best fitted by a
cored King profile in low and intermediate mass systems. The core radius
decreases with cluster mass, and, for the highest mass clusters, the profile is
better represented by a generalized King profile or a cuspy Navarro, Frenk &
White profile. We find a very tight proportionality between $L_{op}$ and
$N_{gal}$, which, in turn, links the cluster mass-to-light ratio to the Halo
Occupation Distribution $N_{gal}$ vs. $M_{200}$. After correcting for
projection effects, the slope of the $L_{op}-M_{200}$ and $N_{gal}-M_{200}$
relations is found to be $0.92\pm0.03$, close, but still significantly less
than unity. We show that the non-linearity of these relations cannot be
explained by variations of the galaxy luminosity distributions and of the
galaxy M/L with the cluster mass. We suggest that the nonlinear relation
between number of galaxies and cluster mass reflects an underlying nonlinear
relation between number of subhaloes and halo mass.
