function onFormSubmit() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Sheet1");
  var lastRowNumber = sheet.getDataRange().getNumRows();
  var cellValue = sheet.getRange(lastRowNumber, 2).getValue();
  if (cellValue === "6") {
    MailApp.sendEmail("youremail@abc.com","Message Subject","Message content.");
  }
}