Technically the most direct form would be "billions" in both cases, but when counting using large quantities as reference in English, you usually say the number of those quantities and then the word that represents them in the singular form. For instance, it would be equally correct to say "six billion people" as "six billions of people", but the former is more commonplace.
I'm not certain that's helpful, since 1. Duolingo often gives us nonsensical phrases to translate; 2. It's totally possible in the future, so a plausible sentence, even considering #1; 3. If I'm not mistaken (and I may be), it could refer to billions of anything: bacteria, square feet of land in the kingdom, bright ideas, etc.
...http://www.huffingtonpost.com/2013/11/29/richest-world-leaders_n_4178514.html Wir mussen das aendern.