Report Builder functions - RowNumber function in a paginated report (Report Builder)
Applies to: 
Microsoft Report Builder (SSRS)
Power BI Report Builder
Report Designer in SQL Server Data Tools
Returns a running count of the number of rows for the specified scope in a paginated report.
Note
You can create and modify paginated report definition (.rdl) files in Microsoft Report Builder, Power BI Report Builder, and in Report Designer in SQL Server Data Tools.
Syntax
RowNumber(scope)
Parameters
scope
(String) The name of a dataset, data region, or group, or null (Nothing in Visual Basic), that specifies the context in which to evaluate the number of rows. Nothing specifies the outermost context, usually the report dataset.
Remarks
RowNumber returns a running value of the count of rows within the specified scope, just as RunningValue returns the running value of an aggregate function. When you specify a scope, you specify when to reset the row count to 1.
scope cannot be an expression. scope must be a containing scope. Typical scopes, from the outermost to the innermost containment, are report dataset, data region, row groups or column groups.
To increment values across columns, specify a scope that is the name of a column group. To increment numbers down rows, specify a scope that is the name of a row group.
Note
Including aggregates that specify both a row group and a column group in a single expression is not supported.
For more information, see Aggregate Functions Reference (Report Builder and SSRS) and Expression Scope for Totals, Aggregates, and Built-in Collections (Report Builder and SSRS).
Code Example
The following is an expression that you can use for the BackgroundColor property of a Tablix data region detail row to alternate the color of detail rows for each group, always beginning with White.
=IIF(RowNumber("GroupbyCategory") Mod 2, "White", "PaleGreen")