From PosiesCafe blog: "When I talked to Lucinda today, she asked if there was a cap on how many were sold to help protect the business from too much loss, and the simple answer is, no. When you sign up for Groupon, you are agreeing to sell as many as get sold… and why would Groupon want it any other way? They get half of the earnings."
From Groublogpon: "Also, to clarify one important point: it has always been Groupon policy to allow merchants to cap deals. If a merchant sells too many Groupons, they’ll have a bad experience, the customer will have a bad experience, and therefore, Groupon loses."