Training
Module
Create formulas that use tables, records, and collections in a canvas app in Power Apps - Training
Do you have need for complex formulas in your app? This module can help you write those formulas.
Report Builder functions - CountRows 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 the number of rows in the specified scope in a paginated report, including rows with null values.
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.
CountRows(scope, recursive)
scope
(String) The name of a dataset, data region, or group that contains the report items to count.
recursive
(Enumerated Type) Optional. Simple (default) or RdlRecursive. Specifies whether to perform the aggregation recursively.
Returns an Integer.
CountRows counts all rows in the specified scope, including rows that have null values.
The value of scope cannot be an expression and must refer to the current scope or a containing scope.
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).
For more information about recursive aggregates, see Creating Recursive Hierarchy Groups (Report Builder and SSRS).
The following code example shows an expression that calculates the number of rows in a row group named GroupbyCategory (based on the expression [Category]).
="Number of rows: " & CountRows("GroupbyCategory")