I will stick my neck out and have a stab ... probably resulting in hoots of derision.
I think the figure is way higher than people would guess I go for ball park 50,000.
Reasoning is if CB+ did not add any more titles and filled in all the gaps on the ones we have then we'd end up with 47,041 books that are recorded on GAC. That is a fact.
Now for long running titles some of these missing books could be from the 60's to I guess 80's. So they are in that figure and will never appear as they are not PD. Plus it includes a couple of UK titles that ran into 1,000+ which are not hosted on DCM. And I would also have to deduct issues that had copyright renewals sitting amongst the PD issues of the same title.
We are now in the realms of guestimates!
So I'll knock off 7,000 for that leaving 40,000.
Now I need to add on books that so far no titles have been added for ... and I go for 10,000.

Giving me 50,000 and I would also say that a lot of these are romance. I have noticed long running romance titles where we just have a couple. So I guess that would indicate we are missing quite a few romance titles as well.
Anyway, that is my guess. Is it anywhere near what other people think? Or am I way off??
Regards,
Mark