Internal count helper.

def _count(self, cmd, collation=None):
        """Internal count helper."""
        with self._socket_for_reads() as (sock_info, slave_ok):
            res = self._command(
                sock_info, cmd, slave_ok,
                allowable_errors=["ns missing"],
                codec_options=self.__write_response_codec_options,
                read_concern=self.read_concern,
                collation=collation)
        if res.get("errmsg", "") == "ns missing":
            return 0
        return int(res["n"])