counter = 0;

function onEdit() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Лист1");
  var basicColumn = 1;
  var idColumn = 2;
  var result = getLastRowAndMaxId(sheet, basicColumn, idColumn);
  var lastRow = result[0];
  var maxId = result[1];

  sheet.getRange(lastRow, idColumn).setValue(maxId + 1);
}

function getLastRowAndMaxId(sheet, basicColumn, idColumn){
  var i = 1;
  var maxValue = sheet.getRange(i, idColumn).getValue();

  while(sheet.getRange(i, basicColumn).getValue() != ''){
    if (maxValue < sheet.getRange(i, idColumn).getValue())
      maxValue = sheet.getRange(i, idColumn).getValue();
    ++i;
  }
  return [i - 1, maxValue];
}