The easist way is to declare a type of array that has only one element, turn range checking off, access the array through the use of a variable, and call the Windows API function Polygon(). Example:

type
  TPtArray = array[0..0] of TPoint;
  PPtArray = ^TPtArray;
procedure DrawDynamicPolyArray(NumPoints : integer;
                               Canvas : TCanvas);
var
  p : PPtArray;
  i : integer;
begin
{$IFOPT R+}
  {$DEFINE CKRANGE}
  {$R-}
{$ENDIF}
  GetMem(p, sizeof(TPoint) * NumPoints);
  Randomize;
  for i := 0 to (NumPoints - ) do begin
    p^[i].x := Random(Form .Width);
    p^[i].y := Random(Form .Height);
  end;
  Polygon(Canvas.Handle, p, NumPoints);
  FreeMem(p, sizeof(TPoint) * NumPoints);
{$IFDEF CKRANGE}
  {$UNDEF CKRANGE}
  {$R+}
{$ENDIF}
end;
procedure TForm .Button Click(Sender: TObject);
begin
  Form .Canvas.Brush.Color := clWhite;
  Form .Canvas.FillRect(Rect(0, 0, Form .Width, Form .Height));
  Form .Canvas.Brush.Color := clRed;
  DrawDynamicPolyArray(SpinEdit .Value,
                       Form .Canvas);
end;
procedure TForm .FormCreate(Sender: TObject);
begin
  SpinEdit .MinValue := 2;
  SpinEdit .MaxValue :=  000;
  SpinEdit .Value :=  00;
end;
end.