Basically if you upload a world to Realms with more files than your mac's max open file limit, rather than stopping you before it uploads, a lot of bad things happen. As expected the world won't upload with the error, "Too many unenclosed archives" or something along the lines of that. More importantly though this will cause other running apps to crash or stop responding at all until a full restart is done, including Safari/Google Chrome/Firefox/Slack/Discord. 

Tl;DR: If # files in world > max open files (default is 12.8k or so), Mac OS fails and causes a LOT of different problems on your machine.