You would want to use a loop to cycle through the rows until you find a row that is blank.

Using excel connector and loop may be time consuming process. Use script to get the count dynamically.

If you still wanted to achieve without script.. you can get started with proxy items. refer https://community1.pega.com/community/product-support/question/pega-robotics-excel-how

Use the ExportToTable method of ExcelConnector and extract proxy which you have DataTable. Now you can get the count of Rows and Columns.