"Germany has eighty million inhabitants."
Translation:Deutschland hat achtzig Millionen Einwohner.
Numbers are generally not capitalised, with the exception of millions/billions/..., because the "main word", so to say, is "die Million" (noun). So it'd be "Die Stadt hat eine Million Einwohner."
Note that million = Million, billion = Milliarde, trillion = Billion, etc.
Since the spelling reform, there's another exception that doesn't make sense to me, but there you are: variations of "hundert" and "tausend" can be capitalised (not necessarily) in these contexts: several hundred trees = ein paar Hundert Bäume (but: 400 trees = vierhundert Bäume); hundreds of people = Hunderte Menschen; they came by the hundreds = sie kamen zu Hunderten; hundreds and hundreds of stars = Hundert(e) und Aberhundert(e) Sterne.
And ordinal numbers are capitalised as well when used on their own, like nominalised adjectives would be: "Der Erste gewinnt eine Million Euro." ("The first [one] (= the winner of the race) wins one million euros."), cf. "Der Dünne zeigt uns den Weg." ("The skinny one/person shows us the way."); but: "der erste Läufer" (the first runner), cf. "der dünne Mann" (the skinny man).