Get Row Count in SSRS Report Builder

Use the CountRows function. For example

=CountRows("MyDataset")

will give you the number of rows in MyDataSet.

As someone else mentioned above, I couldn't get CountRows("DatasetName") to work in the header until I wrapped it thusly:CSTR(CountRows("DatasetName")).

In the Tablix control's properties, there's a property name called NoRowsMessage put your message here when no row is returned.