It's actually a very large universe of people with access to Top Secret data. The Director of National Intelligence publishes what is described as an annual report, "Security Clearance Determinations," although the most recent one I could find was from 2017. In it, more than 2.8 million people are described as having security clearance as of October 2017 " more than 1.6 million have access to either Confidential or Secret information and nearly 1.2 million are described as having access to Top Secret information. There are additional people who have security clearance but don't currently have access to information. This includes civilian employees, contractors and members of the military. Each agency that deals in classification has its own system. Top Secret ... is the highest level of classification. Information is classified as Top Secret if it "reasonably could be expected to cause exceptionally grave damage to the national security," according to a 2009 executive order. A subset of Top Secret documents known as SCI, or sensitive compartmented information, is reserved for certain information derived from intelligence sources. Access to an SCI document can be even further restricted to a smaller group of people with specific security clearances. Information is classified as Secret if the information is deemed to be able to cause "serious damage" to national security if revealed. Confidential is the least sensitive level of classification.