Hello @jlwysf,
Some replies below (caveat: I am also currently learning Rally):
No, client is an Elasticsearch client (they are actually implemented using the elasticsearch-python client).
When you increase the clients, e.g. for a bulk operation, you will see more python3 processes getting spawned.
The following links in docs and discuss replies (1, 2, 3) have more detail.
You should take a look at adjusting clients, as mentioned earlier and running tasks in parallel.
The documentation provides some examples and the amount of clients can be decided by Rally or be specified.
Rally has internal telemetry devices through which it stores a number of very useful metric keys, documented here. Among these metrics, you will a find a number of cpu utilization and disk_io related ones. See also this discuss reply.
Dimitris