There have been estimates of the number of published addresses. For example:
(this chart seems to be underestimating the address count)
However, I'm interested in the number of public keys. Most of them would be obtained from transaction inputs containing P2PKH response scripts.
This article suggests there are at least 56 million, but doesn't describe how this number was found or if the raw data are available:
... I obtained around 56 million pubkeys from the blockchain (mid-January)  ...
Is there a convenient source that gives the number of published public keys in real time?