Is there any way to find out how many filled rows there are in a table?

Yes there is:
please use "GetTableAttribute":

LastRow= Number of the last row of the table (independent of its visualization)

Please note that there can be some "empty" rows at the end of the table. GuiXT returns the number of table rows that are used for scrollbar display. Some of the table rows can have empty values, depending on the application logic. So the "LastRow" value is the maximum number of table lines, but in addition one often has to check that a given row is not empty.