'Return the number of subnets.
The result depends on the identity of
the user making the request (as indicated by the context) as well as
any filters.
:param context: neutron api request context
:param filters: a dictionary with keys that are valid keys for
a network as listed in the
:obj:`RESOURCE_ATTRIBUTE_MAP` object in
:file:`neutron/api/v2/attributes.py`.  Values in this
dictionary are an iterable containing values that
will be used for an exact match comparison for that
value.  Each result returned by this function will
have matched one of the values for each key in filters.
.. note:: this method is optional, as it was not part of the originally
defined plugin API.'

def get_subnets_count(self, context, filters=None):

    raise NotImplementedError()
