You have started DKIM key signing with 2048 bits for your partners due to upgrades in email security standards. You want to know/understand if there's any capping/limits for the total number of domains which can be DKIM signed in Lyris in a single site.
According to architecture of Lyris LM, there's no direct limit hardcoded that can be assigned to DKIM signature.
We have never gotten a report of anyone getting any issues with adding multiple domains. But, if you are about automating it and having hundreds of domains added please consider that it was not tested for that scenario. If you want to assign a higher number of domains (e.g., 500) to a single DKIM signature, due to the absence of a coded limit, it should be possible to configure it. However, the results may vary due to external conditions.
We are unable to directly confirm whether this configuration will work without issues in your environment.
If such configuration results in issues in your environment, you can request assistance in resolving the issues that occur.
Note: Version 12.4 has a known issue related to DKIM that caused mail delivery to halt. The issue was resolved with the release of version 12.4.1, so if you use an older version, it is suggested to upgrade your system to 12.4.1 or later.