Scenario: The requirement is we need ranks based on Value if user enter any number (ex :5 ) report should show top 5 ranks if we enter 20 should be top 20 ranks..

1. Create a report with the columns PROD_NAME, QUANTITY_SOLD and drop QUANTITY_SOLD again and rename it to Rank, and hide this column.
2.  Save the report with the name Rank.

3.  Create a Dashboard prompt with the column QUANTITY_SOLD, and assign a presentation variable  pv_rank  with Edit box list control.
4. Save the dashboard prompt with the name PV_RANK

5.  Write the following formula in the Rank column.

 TOPN(SALES.QUANTITY_SOLD, @{pv_rank})

6.  Create a dashboard page with the dashboard prompt and report.

7.  For example I have taken Rank=10 so the report is, it displays top 10 quantity sold products.