Or “how many as-yet-undiscovered earthquake faults do you believe exist?”, analogous to such often-quoted statistics as “millions of unknown species” in addition to the 1.8 million already catalogued.
The “former cancer survivor” reference in the OP reminded me of an “analogous species” I see disturbingly often – “former graduate” or “former alumnus”. I doubt that many diplomas are rescinded, and I’d guess that even fewer instances exist of people going back in time and erasing the fact that they ever attended a particular school.