Hrm... I wanted to say 10^-7, but then I realized I actually do have evidence that is different from the type of false justifications associated with anosonosia.
The latter, as I understand it, tends to be associated with making excuses for why one isn't moving the arm, etc... I notice though that I actually can move my arms. That is, I observe myself waving both arms freely, typing using both hands, etc.
I'd have to spend a bit more type to work out reasonable numbers associated with it, but these observations are different from the "making excuses for why one can't move the arm in question"
Followup to: The Strangest Thing An AI Could Tell You
Brain damage patients with anosognosia are incapable of considering, noticing, admitting, or realizing even after being argued with, that their left arm, left leg, or left side of the body, is paralyzed. Again I'll quote Yvain's summary:
A brief search didn't turn up a base-rate frequency in the population for left-arm paralysis with anosognosia, but let's say the base rate is 1 in 10,000,000 individuals (so around 670 individuals worldwide).
Supposing this to be the prior, what is your estimated probability that your left arm is currently paralyzed?
Added: This interests me because it seems to be a special case of the same general issue discussed in The Modesty Argument and Robin's reply Sleepy Fools - when pathological minds roughly similar to yours update based on fabricated evidence to conclude they are not pathological, under what circumstances can you update on different-seeming evidence to conclude that you are not pathological?