Reading the question very carefully, I'd have to say "none". If the stack consists of "900 dollar bills", then there are no "50 dollar bills" in it. But the questioner is probably referring to...
One trillion dollars is represented as $1,000,000,000,000.
Divide that by the hundred dollar bill ($100). For those quick minded math people, you drop 2 zeros.
1,000,000,000,000 / 100 =...