Number of used threads/processes for codegen

[burner]

To me it looks like there are always four threads/processes use for code generation.
I can not find the place in the source code where the number is defined, nor could I find a
a cli argument to set that.
I have more than four cpu cores which I would like to utilize, and I can not be the only one.

Where am I being stupid?

edit:
This is an awesome project, thank you.