You can now adjust the number of shards in a sharded keyspace using the new sharded to sharded workflow.
Adding or removing shards from a sharded keyspace requires some rebalancing, as you may be moving data from one MySQL instance to another. This involves creating a new sharded (target) keyspace with the desired number of shards and transferring all of the data from your current (source) keyspace to the new target. This operation is done with no downtime.
For the full tutorial, see the Sharding a sharded keyspace documentation.