You can use the operation /lib/op/filesystem/windows only/excel/Get Row Index by Condition.
You will need to choose a column which will contain the most amount of row data (or loop through all your columns) and use that as the columnIndextoQuery.
Then set operator to != and set value to Use Constant. Add a result field from output field rowsCount.
This will return a count of rows up to the last row found in that column which has data. However, it won't consider any blank cells in that column up to the last row; if you need to count the blank rows as well you will need to use the operation again but this time set operator to be ==. You can then add the two results together.
Hope that helps.