How do you unlock the ability to unlock additional Arcana Cards?

[Atomsk]

Followed this guide and I didn't unlock the extra Arcana. I have all the DLC but the Emergency Meeting one.

[Moonbane]

Are you talking about the basic set of arkana cards or the new darkanas?

I suspect you are trying to unlock the darkanas. You need to get the Darkasso relic first,
https://vampire-survivors.fandom.com/wiki/Room_1665