As to how many planes, we need to figure out how much cash and how much it weighs. I did some quick math, and if my info is close, 250 billion usd using 500 Euro notes would be somewhere between about 400-600 tons (depending on the exchange rate).

1.21g - weight of 500 euro note

2.1 - pounds per kg

(((((250,000,000,000 / 500) * 1.21) / 1000) * 2.1) / 2000) * exchangerate

Swiss francs were also used, but this should at least give us an order of magnitude.