In a future major version, we will be introducing a soft limit on the
number of shards in a cluster based on the number of nodes in the
cluster. This limit will be configurable, and checked on operations
which create or open shards and issue a warning if the operation would
take the cluster over the limit.

There is an option to enable strict enforcement of the limit, which
turns the warnings into errors.  In a future release, the option will be
removed and strict enforcement will be the default (and only) behavior.