procedure TForm1.Button12Click(Sender: TObject);
  begin
  if savedialog1.Execute() then
    ListBox.items.saveToFile(saveDialog1.fileName)
  end;

procedure save250( filename: string; var strs: tStrings; startAt: integer );
  var
    sl:   tStringList;
    b, e: integer;
  begin
  // Our temporary string list
  sl := tStringList.create;
  // indices of lines to copy, inclusive
  b  := startAt;
  e  := startAt + 249;
  if e >= strs.count then e := strs.count -1;
  // copy the indexed lines into our temporary string list
  for b := b to e do
    sl.append( strs[ b ] );
  // save the lines to file
  sl.saveToFile( filename );
  // cleanup
  sl.free
  end;

procedure TForm1.Button12Click( Sender: TObject );
  var
    lines_per_block: integer;
    line_index:      integer;
    file_number:     integer;
    file_extension:  string;
    base_filename:   string;
  begin
  lines_per_block := strToInt( edit6.text );  // number of lines to save per file
  if saveDialog1.execute then
    begin
    // get the file extension and everything except the extension in
    // separate strings so we can inject the 1, 2, 3, ... file number.
    file_extension := extractFileExt( saveDialog1.filename );
    base_filename := copy(
      saveDialog1.filename,
      1,
      length( saveDialog1.filename ) -length( file_extension )
      );
    // save each block of lines
    line_index  := 0;
    file_number := 1;
    while line_index < ListBox.items.count do
      begin
      save250(
        base_filename + intToStr( file_number ) + file_extension,
        ListBox.items,
        line_index,
        lines_per_block
        );
      inc( file_number );
      inc( line_index, lines_per_block )
      end
    end
  end;