As of this writing, The Da Vinci Code by Dan Brown is the number one bestselling book of all time. Likewise, it is the top most bestselling novel of all time. It recorded more than 5.09 million volume sales.
Three Harry Potter novel books by J.K Rowlings ranked 2nd, 3rd and 4th. The 5th place went to Fifty Shades of Gray by E.L. James with more than 3.75 million volume sales.
So, if you want to write a book that can surpass the volume sales of The Dan Vinci Code, you must be able to sell at least 5.1 million copies.