code: def num_machines(self, machine_type=None, dataset=None, state=None, 
              memory=None, tombstone=None, tags=None):
          
          _, r = self.request('HEAD', '/machines')
          num = r.headers.get('x-resource-count', 0)
          return int(num)
docs: ::
          
              HEAD /:login/machines
          
          Counts the number of machines total, or that match the predicates as
          with :py:meth:`machines`.
          
          :param machine_type: virtualmachine or smartmachine
          :type machine_type: :py:class:`basestring`
          
          :param dataset: unique ID or URN for a dataset
          :type dataset: :py:class:`basestring` or :py:class:`dict`
          
          :param state: current running state
          :type state: :py:class:`basestring`
          
          :param memory: current size of the RAM deployed for the machine (Mb)
          :type memory: :py:class:`int`
          
          :param tombstone: include machines destroyed in the last N minutes
          :type tombstone: :py:class:`int`
          
          :param tags: keys and values to query in the machines' tag space
          :type tags: :py:class:`dict`
          
          :Returns: a count of the number of machines (matching the predicates) 
              owned by the user at this datacenter
          :rtype: :py:class:`int`