def num_machines(self, machine_type=None, dataset=None, state=None, <EOL>memory=None, tombstone=None, tags=None):
_, r = self.request('<STR_LIT>', '<STR_LIT>')<EOL>num = r.headers.get('<STR_LIT>', <NUM_LIT:0>)<EOL>return int(num)<EOL>
::

    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`