according to http://keys.mayfirst.org/pks/lookup?op=stats there's currently 4594571 keys on the bulk of public keyservers. considering the rumors that facebook and others also do signal and their user base is around a rumored billion, the k that i glossed over is around 217. even if we assume that there's dark masses that never ever used a keyserver, we ignore the fact that out of those 4.5 million pgp keys most are expired, revoked or simply lost, so the active keys on the keyservers are probably much less, and thus k is also much bigger.