Default: half the available CPUs on the machine.

My command

npx loadtest -H ... -t 5 -c 20 http://localhost:8080/xxx

My output

Max time (s):        5
Concurrent clients:  120
Running on cores:    6
Agent:               none

Completed requests:  162
Total errors:        0
Total time:          6.364 s
Mean latency:        2112.9 ms
Effective rps:       25

I see that it utilizes all available (6) cores, instead of 3.

Also, for me, it looks like concurrency is taken per core, not per process. So total clients will be CORES*CONCURENCY

Loadtest v8

@alexfernandez If it's correct, I can update the doc