There is (or appears to be, though I haven't found it documented anywhere) a 64 (concurrent) child limit to the win32 fork emulation. This is probably due to the use of WaitForMultipleObjects(Ex) to implement wait and/or waitpid which has a maximum number of "objects" it can wait on WinNT.h:#define MAXIMUM_WAIT_OBJECTS 64     // Maximum number of wait objects

This limit could be lifted, as it has been with threads (win32 forks are threads underneath).