How can I call a DataService with an out parameter in DataSource

I can't answer your question about outbound parameters, but the following should get you the counts on the number of records returned.

<label id="lblCount">
<bindings>
<binding dataContext="dataSource" dataPath="rowCount" property="text"/>
</bindings>
</label>

yes,but the rowCount of dataSource only is the pagesize number not the fact rowcount in database

I see, why not just call another webservice that returns that count for you? I doubt there'd be much of a performance hit doing that.

I suppose you are trying to accomplish something like "Showing 1-10 of 236 records."?