I'm not sure if hw.availcpu is the right value to use as it is not documented at all (neither in a manpage, nor in a headerfile).

hw.activecpu seems to be the one that should be used: it is documented as "The number of processors currently available for executing threads." in the sysctl.h header file and that comment also mentions that it should be used to determine the amount of threads to start in an SMP application.