I have been reading every public comment on the Sun Certified Java Programmers Exam since 1998, and I have never seen any reliable information on this. It would be fun/interesting to know, but this information has never been made available. All we have are informed guesses.
I go with Bill on this one. One book claimed to have sold over 100,000 copies on the first run and it is now into its third or fourth edition and there are several other books that compete strongly with it. Even if say only 1 in three purchasers actually pass the exam the numbers must be in the hundreds of thousands. Lies dammned lies and statistical guesses.
Also consider that many people will purchase more than one book and you lower the numbers significantly.
If a million certification books have been sold, on average people purchase 2 books and a third of the purchasers actually take AND PASS the exam you're looking at 1/3 of half a million people certified. That's rougly 165.000 people worldwide per million books sold.