Comment 1 for bug 1263540

I saw the following while attempting to work around this issue:

1.
In addition to /etc/apt/trusted.gpg, each *.gpg file in /etc/apt/trusted.gpg.d/ is a separate keyring, often containing a single key for the corresponding repository. This could effectively limit the number of repos/packages one can have, if the total number of keyrings exceeds GnuPG's limit.

2.
Deleting a key ('apt-key del <keyID>'), or removing a repository (e.g., using Synaptic), removes the key from its keyring in /etc/apt/trusted.gpg.d/ but leaves the empty keyring in the location. After I removed the empty keyring files, the "resource limit" message did not appear and 'apt-get update' did not complain about "NO_PUBKEY." So, once GnuPG's maximum number of keyrings is reached, one has to manually remove the empty keyring files, in addition to removing package repositories, in order to avoid the "NO_PUBKEY" scenario.