I got “a few million” from this LA Times article. It says ‘About 4.2 million adults receive monthly benefits that are managed by “representative payees.”’, which is the number I was vaguely recalling when I posted. To be open about the discussion, later on the article says, ‘About 2.7 million people are now receiving disability payments from Social Security for mental health problems, a potentially higher risk category for gun ownership. An addition 1.5 million have their finances handled by others for a variety of reasons.’ so perhaps I should have said “1.5 million” instead of a “a few million”.
As for “prohibited persons”, while the WhiteHouse.gov “fact sheet” is admittedly vague on the details, it’s my understanding that they’re going to report the identities of these people to NICS, probably as “mentally defective”. Reporting the identities of people to NICS is, as I understand it, what makes them a “prohibited person” (for firearm purchases). Is there some other definition of “prohibited person” you were working from?