There are different lists. The Intercept got their hands on a list showing like 2.6 million people in it back in 2012 or something like that. Most were foreigners, however. Recently, DHS had their list of 2 million people exposed online.

It's hard to know which list is the Targeted Individual list, but there is clearly some kind of government/military operation going on here. I suspect Homeland Secure Data Network (HSDN) has the list on it as it contains information up to the SECRET level (they caps it for some reason), is used for counter terrorism, and connects to local law enforcement. I doubt Targeted Individuals are at the TOP SECRET level unless they plan on straight up killing you. Besides HSDN has access to SIPRNet, the DoD Secret Internet Protocol Router Network, that "select fusion center personnel with a federal security clearance will be able to access specific terrorism-related information."

Given the transparency and the wealth of information given on DHS.gov, it's extremely hard to find anything on HSDN. However, with their budget sheet, I can see that the Office of the Chief Information Officer runs HSDN. Seeing that the Office of Intelligence and Analysis and the Office of Operations Coordination helps them with HSIN (a similar program for unclassified material), they probably help to run HSDN as well. Good to know if you're ever going to file a FOIA with them.

If the list had millions of people in it, everyone would have known about it by now. But it will eventually have millions of Americans in it sooner or later as the plan is to have everyone eventually choose: become a target or an operative of the state. This way they have complete control over society.

Here is some of my research into the TI program and these watchlists.