def _set_cores(self):
        """set cpu numbers to be used"""

        cpu_num = cpu_count()-1
        if self._processes > cpu_num:
            cores = cpu_num
        elif self._processes < 1:
            cores = 1
        else:
            cores = self._processes
        # have to be int if user input is float
        self._processes = int(cores)
set cpu numbers to be used