I'd put that calculation in the report, in the Report Footer section.
Open the Report Footer section, add the field you want to count. Right click the field, choose Properties, on the All tab scroll to Running Sum. Open the drop down box and choose how you want the field to sum (Over All or Over Group).
To manually add a Running Sum field, add a Text Box, right click over it and choose Properties. The second field under the All tab is Control Source. Click the "build" button to the right to open the Expression Builder. The command is "count". I'd probably count the Autoid field, making the expression count[autoid]. You can count any text field in the detail line, though.
Hope that helps!