It depends on many factors what the optimal number of concurrent builds is, but the bottom line is that you want to maximize your CPU utilization and minimize context switching.

If you think that one extra concurrent job is enough to fill CPU utilization in the time that other jobs are blocking on iowait, then you are fine.

So, bottom line, factors to think about:

- your i/o throughput for writing the generated object files;

- the complexity of the code being compiled, - template-rich C++ code has a lot higher CPU usage versus i/o ratio

- the amount of cores in your system