var
  slSource, slSave: TStringList;
  i,j: integer;
begin
  slSource := TStringList.Create;
  slSave := TStringList.Create;
  try
    slSource.LoadFromFile('C:\test.txt');
    j := 1;
    for i := 0 to slSource.Count -1 do begin
      slSave.Add(slSource.Strings[i]);
      if (i mod 100 = 99) or (i = slSource.Count -1) then begin
        slSave.SaveToFile('C:\test'   IntToStr(j)   '.txt');
        slSave.Clear;
        Inc(j);
      end;
    end;
  finally
    slSave.Free;
    slSource.Free;
  end;
end;