Are you planning to fetch the number of assignments present in work queue?
You can explore this RD
This is used to fetch the current user's work basket. This is the source for D_WorkBasket data page. Parameters to the report definition are passed by the load activity of the data page. Do not use this to get the work basket; use the data page instead.
To use RD as for WorkBasket, you can use Pega's OOTB ReportDefinitions for fetching the WorkBasket Data.
Use the above Report Definitions as a source in a Table or Repeat Grid and pass the Parameter values and configure it. You can get WorkBasket Data.
If you want to customize the Report Definition, you can Save As it to your Application ruleset and apply your customizations.