Total US M2 money supply was $11.269 TRILLION on 2014-05-12.
http://research.stlouisfed.org/fred2/series/M2/
That's 536.62 times as much as eventual DGB.
And it's still worth a Dollar!
21 billion is puny in comparison (especially when taken in a worldwide context where the combined M2 of industrial nations is around 6 times that of just the US). If anything, one might argue that 21 billion borders on the low end.
I suggest you don't get overly excited about those numbers. You need to think first what is a realistic market share for DigiByte out of all that. And if you come up with anything larger than 0.001%, I will call you delusional.