2004: 15.6 million (citation)
2008: 23.4 million, also the highest year of the Bush presidency. (See table on page 9 (the 13th actual PDF page) from the 2009 report from the Information Security Oversight Office)
As dismal as was Bush's record on secrecy, Obama's is worse, hitting nearly 77 million pages in 2010, up from 54 million in 2009 (see table on page 9 (the 13th actual PDF page) from the 2009 report from the Information Security Oversight Office). To be fair, the chart does indicate that the massive increase in the number of classifications under the Bush administration to the Obama administration resulted from a change in counting procedures. However, the increase alone between 2009 and 2010 was 23 million documents. Because 2009 and 2010 have the same counting rules, this demonstrates that rather than reducing the number classifications, Obama is accelerating classification.
The Electronic Freedom Foundation has compiled an extensive list of Obama's secrecy failures in its article: 2011 in Review: The Year Secrecy Jumped the Shark (fn1). Some of the EFF's list items are amusing, in a bitter black humor sort of way, e.g.: "President Obama accepts a transparency award … behind closed doors"; or 'Headline from the Wall Street Journal in September: "Anonymous US officials push open government."' The EFF is not alone in noting Obama's disturbing rush toward greater secrecy. Here's more of Obama's "worse" and "worse".
As an interesting side note regarding classification of documents and government secrecy, consider the history of the State Secrets Doctrine, an argument Obama has widely used in court cases in order to protect torturers and illegal wiretappers. The State Secrets Doctrine achieved its modern incarnation in a case involving the Air Force's refusal to turn over airplane crash documents in a wrongful death lawsuit in the 50s. The case went to the Supreme Court which set the policy in stone, refusing to let the widow have the documents in her attempt to prove the government was negligent. 40 years later, when the documents finally saw the light of day, the only secret they contained was the fact that the government failed to perform required maintenance and suggested manufacturer upgrades on the plane that crashed. In other words, the government used the classification process to avoid being held liable for negligently killing a man, not to protect state secrets.
For more information, see (or rather listen to) This American Life: Episode 383: Origin Story, Act 2; Book by Pulitzer Prize winning author Barry Siegal: Claim of Privilege.
In any event, with 77 million documents being classified by the Obama administration, that's 2.44 documents per second 24/7, it is a sure bet that a large number merely conceal government errors or malfeasance.
(fn1) Jumping the Shark: a term to describe a moment when something that was once great has reached a point where it will now decline in quality and popularity. Origin of this phrase comes from a Happy Days episode where the Fonz jumped a shark on waterskis. Thus was labeled the lowest point of the show." Jumping the shark video clip.