Other mistake: At the start of the movie it shows the house at front view and the sky is very very grey, it looks like it's about to rain, then after a short scene inside the house it goes back outside and the sky is clearly blue with not a cloud in sight. (00:12:00)
Other mistake: When the $50k check is revealed about Clarence Beeks, both Mortimer and Randolf's positions and their angles appear as if Winthrop is behind them, not in front. (00:15:00 - 00:17:00)
Other mistake: After Winthorpe exits the bus and is wearing his Santa outfit, a dog urinates on his leg. Not only is there no "stream" of urine from the dog, but there also is no evidence of urine on the ground after the dog finishes his business and trots along. (01:21:25)
Answer: Without knowing precisely how many stocks Winthorp and Valentine sold at the beginning, there is no way to get an exact figure. But we can make some educated guesses. Frozen Concentrated Orange Juice is sold in contracts, with each contract containing 15,000 pounds. By the end of the day, Winthorp and Valentine were making at least an 84 cent profit on each pound, and $12,600 on each contract. If they sold 20,000 contracts, their profit would be $252 million. Considering the Duke's lost $394 million, that estimate seems to fit (and is likely even a little low).