On Macs, this "total cores available" strategy might not be a valid approach, since you probably only want to be using the high-performance cores and not also the low-power ones — and the CPU might well overbook all your threads to high-perf to leave room for the OS and other programs, even if you try to do otherwise.

Not just Macs, plenty of Linux systems have a big.LITTLE design.

with a big.LITTLE system it'll really depend what you're doing. Just compiling something? It's probably fine (if all cores can be active at once). If you're wanting more consistent performance for some calculations? You might need to think a bit more.

Another thought, I don’t know if Alpine Linux or other musl-libc based systems will have ‘nproc’ if, for instance, you’re running containers or other minimal systems.

cpuset -g on FreeBSD >= 7.1 will tell you what cpus are available to your shell, where a sysctl will only tell you what's on your system. (Lots of good info in the related manual pages)