"Historien berättad från den symbiotiska organismens synvinkel medan du bor i människor" låter som 'Nåle' av Hal Clement , publicerad 1950 , men jag kommer inte ihåg någon "identifiering av människor efter antal öppningar" från den historien.
Sammanfattning från Wikipedia:
The Hunter, an alien lifeform (when not inside another being, resembling a four-pound green jellyfish) with the ability to live in symbiosis with and within another creature, is in hot pursuit of another of his kind. Both crash their ships into Earth, in the Pacific Ocean, and both survive the crashes.
The Hunter makes its way to shore (its erstwhile host having been killed in the crash) and takes up residence in the nearest human being it can find (as it turns out, fifteen-year-old Robert Kinnaird) without letting the human being know. By the time it has figured out enough of what goes on inside a human being to look through Bob's eyes, it is shocked to find itself within an air vessel, being carried further away from its quarry every second. As it happens, Bob is simply returning to a New England boarding school from his home on an industrial island in the Western Pacific.