I think you must be running a loop for displaying items. In that loop only take an integer variable and increment the variable on loop ending .The variable will return accordingly to the number of items displayed and take a new field in the report which will show you the exact number of items.
Initialize the variable in your fetch method of the report and increment that variable under your main loop of fetching items and write the display method as follows :
As said by Lalit it can be done by incremeting the variable in the fetch method and having the display method which return the incremented value.
The other way is, just declare the variable in the class declaration, and have the same display method in the \Reports\Report1\Designs\ReportDesign1\AutoDesignSpecs\Body:\Methods - as