Darth, I've just checked your link and the calculator suggests that 1,000,000 is expressed as MMMMM.......(one thousand times). A little impractical for your average Roman. I offer another link, which, if you scroll down the page to Roman Numerals, agrees with Priveet's answer. http://www.math.com/tables/general/numnotation.htm
M with a bar over it. Pronounced mille milia in digit form (example - one, two, three...), and mille millesimus in ordinal form (example - first, second, third...). See the Roman numerals chart at https://www.unrv.com/culture/roman-numerals.php