I tried that and I got the following error:

A group expression for the list ‘list1’ uses the RowNumber function with a scope parameter that is not valid. When used in a group expression, the value of the scope parameter of RowNumber must equal the name of the group directly containing the current group.

My understanding is that the GROUP ON EXPRESSION is the definition of the group, so it defines the scope. So where do I add the expression =CInt(Ceiling(RowNumber(Nothing)/25))?

The specific steps I take are Edit Details Group Properties of List Box -> Group ON Expression = ?