When creating new enumerations it can be useful to the client to know 
(prior to receiving the data) how much data will be coming so it can 
allocate the proper space.  To that end, it would be nice if client could 
ask for the EnumerateResponse message to include this information.

Proposal:

Extend the Enumerate request with the stuff in bold:
    <wsen:Enumerate ?>
      <wsen:EndTo>endpoint-reference</wsen:EndTo> ?
      <wsen:Expires>[xs:dateTime | xs:duration]</wsen:Expires> ?
      <wsen:Filter Dialect="xs:anyURI"?> xs:any </wsen:Filter> ?
      <wsen:RequestCount Exact="xs:boolean"?> ?
      ?
    </wsen:Enumerate>

/RequestCount
  when present this element indicates that the client is asking for the
  data source to return the number of data items in the EnumerateResponse 
message.
  When not present the default value is value. If the service doesn't 
support
  this option then a fault must be generate.
/RequestCount@Exact
  when present, and set to 'true', this attribute indicates that the data 
source MUST return
  the exact number of items in the filtered enumeration in the 
EnumerateRespones. If an exact count
  can not be determined then a fault must be generated. The default value
  is 'false' - which means the service can choose to estimate the total 
count
  if that is all it can do.

and the EnumerateResponse with:
    <wsen:EnumerateResponse ?>
      <wsen:Expires>[xs:dateTime | xs:duration]</wsen:Expires> ?
      <wsen:EnumerationContext>?</wsen:EnumerationContext>
      <wsen:TotalCount Exact="xs:boolean"?> xs:long </wsen:TotalCount> ?
      ?
    </wsen:EnumerateResponse>

/TotalCount
  when present this element indicates the total size of the filtered
  enumeration. This element MUST be present if the RequestCount
  element was on the Enumerate request.
/TotalCount@Exact
  when present and set to 'true' this element indicates that the total 
count value is
  the exact number of items in the filtered enumeration and not an
  estimate.  The default value is 'false'.