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You may use OnKeyPress event of your Edit. You should check, if key parameter is not in set '0'..'9' then you must write Key:=#0;

var
  NumSet: set of Char = ['0'..'9'];

procedure TForm1.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
  if not (Key in NumSet) then
    Key:=#0;
end;