  We characterize the degree of entanglement of a subsystem of $k$ particles in
a $N$-two level system ($k\leq N/2$) initially prepared in a mesoscopic
superposition $|\psi>=\int d\theta f(\theta) (|\phi_{1}(\theta)>^{\otimes
N}+|\phi_{2}(\theta)>^{\otimes N})$, where $f(\theta)$ is a gaussian or a delta
function, subject to the time evolution described by a dephasing channel.
Negativity is used as a measure of entanglement for such system. For an
arbitrary number of particles $N$, numerical results are given for the full
time evolution up to ten particles. Analytical results are obtained for short
times and asymptotic time regimes. We show that negativity is initially
proportional to the square root of the product of the number of particles in
each partition, the overlap ${|<\phi_1(\theta)|\phi_2(\theta)>|}^2$ and the
coupling to the environment. Asymptotically, negativity tends to zero, a
necessary condition for separability.
