  Massive MIMO is a promising technique to increase the spectral efficiency of
cellular networks, by deploying antenna arrays with hundreds or thousands of
active elements at the base stations and performing coherent beamforming. A
common rule-of-thumb is that these systems should have an order of magnitude
more antennas, $N$, than scheduled users, $K$, because the users' channels are
then likely to be quasi-orthogonal. However, it has not been proved that this
rule-of-thumb actually maximizes the spectral efficiency. In this paper, we
analyze how the optimal number of scheduled users, $K^\star$, depends on $N$
and other system parameters. The value of $K^\star$ in the large-$N$ regime is
derived in closed form, while simulations are used to show what happens at
finite $N$, in different interference scenarios, and for different beamforming.
