That number doesn't match what the SSA publishes in their annual reports.[1]. They put the figure on disability at 8.9 million disabled workers, with 11.2 million total beneficiaries once you include the disabled workers' children.)
That said, the real figures are still startling. They represents 4.4% of the US worker population but some states rely incredibly heavily on the program.
Alabama - 8.0% of the population
Arkansas - 7.9% of the population
Kentucky - 7.6% of the population
Maine - 7.4% of the population
Mississippi - 7.5% of the population
West Virginia - 8.6% of the population