You can try "Sub-file/folder count" though that will match the folder. If you want to match the file within, you would first need to create a rule to go into subfolders (search for it in the help). Next you need another rule to match the file.
Now there are several ways to do this. If you set up your subfolder rule to only go into folders with one item in them, then the rule to match the file can just move whatever it finds. It gets more complicated if you need to set up your subfolders rule to go into the folder for other reasons.