Limit the number of backup folders

gdrive-folder-limit.gs

function deleteOldSheets() {
  var df = DriveApp.getFolderById("your-backup-folder-id");
  var arr = new Array();
  var folders = df.getFolders();
  while( folders.hasNext() ) {
    var f = folders.next();
    arr.push( [ [ f.getId() ] , [ f.getLastUpdated() ] ] );
  }
  arr.sort( sortFunction );
  
  for( var i=14 ; i<arr.length ; i++ )
    DriveApp.getFolderById( arr[i][0] ).setTrashed( true );
};

  function sortFunction( a , b ) {
  var aDate = new Date(a[1]);
  var bDate = new Date(b[1]);
  if ( aDate === bDate ) return 0;
  else if ( aDate < bDate ) return 1;
  else return -1;
};