Harry VI's first day: 8.9 million

The sixth volume of the Harry Potter saga sold more than 8.9 million copies in the first 24 hours it went on sale in the United States and Britain to become the fastest-selling book in history, publishers said.

"British book chain WH Smith reported sales of 13 books per second across the 391 shops it opened into the early hours of Saturday, compared with eight per second for the fifth Harry Potter adventure."

seems publisher had full preparation for the first day sale, if they didn't print enough copies, then people would have to wait for the back order. They had foreseen the huge demand in advance.