You can not use a dynamic number for that value of Rows…what you can do is untick the box for fixed number, so that it will in essence be dynamic, which simply means it will have as many rows as necessary based on the datasource and number of results returned.
Then in your datasource, you can use operators for items from and items until in order to show only a certain number of entries based on a dropdown value.
However, if the goal is for pagination, there is much more work to be done to set that up properly.