But that's /sys, which is also rather more mutable than the syscall interface.
getconf _NPROCESSORS_CONF (or _NPROCESSORS_ONLN) is the thing to use (or, from a program, sysconf(_SC_NPROCESSORS_CONF) et seq. This is in glibc, so has a stability guarantee that is worth something. Let it figure out where to go for the answer.