Many millions, in my opinion. The reason is that I’ve been ripping CDs to FLAC for a slimserver installation. I came across quite a few CDs showing signs of corrosion, the notorious “CD rot”. Almost all are made by PDO. The problem seems to strike the outer rim of the label side first, which goes golden brown. The brown gradually spreads across the whole side, eventually being clearly visible on both sides of the CD. The affected CDs seem to cover quite a wide time span in date of manufacturer, roughly 1989 – 1994 according to my impressions. Wikipedia says 1988-1993 in a rather good article on the subject – however, it has no estimate of how many CDs were affected.
The good news is that so far all my bronzing CDs still play. Now that they are ripped to a lossless format, at least I have a backup. I presume at some point they will become unplayable.
In a way, I guess PDO has got away with it. Yes, they are faulty CDs, but even a 15 year life is not that bad. By the time they fail in their millions, probably most people will be playing music from files rather than CDs.