Rule for if folder only has one item

I want a rule that does one thing if a folder has one mp3, and something else if there is more than one. There might be other files in the folder, but what matters is the number of mp3s.

What do you want it to do in that case?

If there's one mp3, I want to open it in Switch. If there are multiple, I'd like it to open Audiobook Builder

You can try using a nested condition (look it up in the manual). You could have it match a file if there are other files in the same folder that are mp3s as well.