  Gravity and matter are universally coupled, and this unique universality
provides us with an intriguing way to quantifying quantum aspects of space-time
in terms of the number of gravitons within a given box. In particular, we will
provide a limit on the number of gravitons if we trace out the matter degrees
of freedom. We will obtain the universal bound on the number of gravitons,
which would be given by $N_{g}\approx(m/M_{p})^{2}$. Since the number of
gravitons also signify the number of bosonic states they occupy, the number of
gravitons will place an indirect constraint on the gravitational entropy of the
system. We will show that it saturates Bekenstein bound on the gravitational
Area-law of entropy. We will also find that our conclusion is quite robust
against the initial state of the matter degrees of freedom. Based on these
observations, we will ascertain that the gravitons permeating in the observable
Universe always $N_g\gg 1$.
