How many people have been diagnosed with sporadic fatal insomnia?

Very very few: <100 around the world & over time. You don't have it. Btw, it is closely related to mad cow disease & creutzfeldt-jakob disease & is caused by accumulation of misfolded prion proteins in the brain.

Ultra few: I do not know, but do know it is so incredibly rare. Something like less than 10 sporadic cases, and only a few dozen families with the abnormal gene.