I don't know. I subscribe to a few threads that I delete one or two at a time, when they are closed or deleted. The same is true about the members I subscribe to.
Members have posted that they get error messages when they try to delete "too many." What have you found out?
I get failures somewhere north of 10, but don't know exactly where. I usually wait till there's a pile I'd like to delete, and therein lies the problem