const value=workbook.sheet("Sheet1").usedRange().value().length;
usedRange () - pulls all values from the sheet. Thus we find how many lines are already taken. Of course, this will be the last line if all the found lines are filled, so the answer is not complete
$objPHPExcel->setActiveSheetIndex(0)->getHighestRow();. I did not find an analogue in the xlsx-populate package. TS, I think you need to search by line by line search - Total Pusher