I have a big server I am making and there will be both "apartments" and "hotel rooms" using the keycard system. However, I am wondering how quickly I would hit the limit (currently it's 32768), and I don't really see a technical reason for the limit at all. Can you please make it so we can set a maximum in the config file (e.g. somewhere between 32768 and 99999) or increase it to 65336?

The limit was chosen because it is 2^15 (maximum value for the short type in Java), and we didn't think anyone would ever have the need for more than thirty-two thousand keycard setups. But we can easily increase the limit to 99999.

Is this going to be in 1.16 as well? I forgot to mention the version and it's an easy enough change.

Yes. All gameplay changes/features/fixes we make are done on all versions that we currently support, which are currently 1.12.2, 1.16.5, 1.18.2, 1.19.3, and 1.19.4. Commit 1acee12 is for 1.16.5.