Your i7-7700k has only 8 threads, so this is expected.
The r setting is actually for “pairs” of threads, and there’s also helper threads in addition. If you’re running linux, you can see this by running top and press “H” to show threads.
For example, running -r 20 right now, I have 20 “phase1/eval” threads, 20 “phase1/match” threads, and 5 “Disk/read” threads, and a few others like phase1/slice etc. There’s a total of over 70 threads from running -r 20.
If you want to use less than 100% cpu, you need to choose a lower number than -r 4.