Banter and General Comments, page-13589

I recall @tutor mentioning that they can be lent multiple times (or something like that), and when the aggregate number was about 60ish million, iirc he said that the net number was 40-something million. And apparently if they are lent they are usually shorted also, otherwise the borrower holds them for a fee without putting them to work...
So, it seems there are actually about 40 million shares lent and shorted. Still shedloads. But yes, who knows exactly how many would need to be covered to bring it back to zero...
They like it mysterious...