Thank you for writing.
I am glad that you have found a suitable solution for your requirement. You can refer to our GridView >> Formatting Group Header Row
help article which is quite useful on this topic. The RadGridView.GroupSummaryEvaluate
event allows you to modify the header text of the group rows. The event is fired when the group header row text is needed. Hence, you can include the rows count as well.
I hope this information helps. Should you have further questions, I would be glad to help.