Easy.
Add a text box and for it's value use this expression:
=COUNT(Fields!{some non-nullable field from your dataset}.Value, "{your dataset}")
"I cant stress enough the importance of switching from a sequential files mindset to set-based thinking. After you make the switch, you can spend your time tuning and optimizing your queries instead of maintaining lengthy, poor-performing code."-- Itzik Ben-Gan 2001