MergeSpreadSheetAlgorithm.js

function mergeIntoMainSheet() {
  let spreadSheet = SpreadsheetApp.getActiveSpreadsheet()

  /**
   * Inserisci il nome della tabella dove vuoi importare tutti i dati
   */
  let mainSheet = spreadSheet.getSheetByName('Tabella Totale')

  /**
   * Inserisci tutti i nomi dei clienti da cui vuoi copiare i dati e inserirli nella tabella principale definita sopra
   */
  let clientsSheet = ['Alessandra Gatti', 'Mattia Pasquetto']

  for(let i = 0; i < clientsSheet.length; i++) {
    let client = clientsSheet[i]
    let clientSheet = spreadSheet.getSheetByName(client)
    copyDataIntoMainSheet(i, mainSheet, clientSheet, client)
  }
}

function copyDataIntoMainSheet(i, mainSheet, clientSheet, clientName) {
  var margin = 0
  if (i > 0) margin = 4
  let availableRow = getBusyRowsNumber(mainSheet)
  let clientRange = getDataRangeFromClientSheet(clientSheet)
  mainSheet.getRange(availableRow + margin, 1).setValue(clientName.toUpperCase())
  mainSheet.getRange(availableRow + margin + 1, 1, clientRange.getNumRows(), clientRange.getNumColumns()).setValues(clientRange.getValues())
}

function getDataRangeFromClientSheet(clientSheet) {
  return clientSheet.getRange(2, 1, getBusyRowsNumber(clientSheet), getBusyColumnsNumber(clientSheet)) // row, cloumn
}

function getBusyRowsNumber(sheet) {
  return sheet.getDataRange().getLastRow()
}

function getBusyColumnsNumber(sheet) {
  return sheet.getDataRange().getLastColumn()
}

/**
 * Only for test purposes
 */
function testBusyMethod() {
  let spreadSheet = SpreadsheetApp.getActiveSpreadsheet()
  let sheet = spreadSheet.getSheetByName('Alessandra Gatti')
  console.log(getBusyColumnsNumber(sheet))
}