  Massive MIMO is a promising technique to increase the spectral efficiency
(SE) of cellular networks, by deploying antenna arrays with hundreds or
thousands of active elements at the base stations and performing coherent
transceiver processing. A common rule-of-thumb is that these systems should
have an order of magnitude more antennas, $M$, than scheduled users, $K$,
because the users' channels are likely to be near-orthogonal when $M/K > 10$.
However, it has not been proved that this rule-of-thumb actually maximizes the
SE. In this paper, we analyze how the optimal number of scheduled users,
$K^\star$, depends on $M$ and other system parameters. To this end, new SE
expressions are derived to enable efficient system-level analysis with power
control, arbitrary pilot reuse, and random user locations. The value of
$K^\star$ in the large-$M$ regime is derived in closed form, while simulations
are used to show what happens at finite $M$, in different interference
scenarios, with different pilot reuse factors, and for different processing
schemes. Up to half the coherence block should be dedicated to pilots and the
optimal $M/K$ is less than 10 in many cases of practical relevance.
Interestingly, $K^\star$ depends strongly on the processing scheme and hence it
is unfair to compare different schemes using the same $K$.
