Is there an efficient way to obtain the number of seats utilized in your org? There's the "Get Enterprise Users" call, but it appears to return all your users in paginated form. Sounds like an expensive way to just get a current user count.
The paginated result returned by Get Enterprise Users includes a total_count field that indicates the size of the collection. You can request a small subset of the collection (offset=0&limit=1) to speed up the request.