This is not currently supported. Unlike BOSS which works from a single index/database, one of the reasons its very difficult to come up with a number is because YQL combines remote filtering with local filtering. Sometimes remote services provide a number of results, sometimes they dont. This number of potential results is further made difficult to determine because the "local" parts of the WHERE clause will change that number.
However, YQL, by default, returns ALL available results from your query. If you'd like to page through results you can use OFFSET and LIMIT.