Physical books peaked at around 4000, but over the course of a few moves over the last decade and a half have dropped to maybe 1500 or so now.

Ebooks, I really have no idea. A friend gave me a couple of large collections - well into four figures - some years back and I have acquired quite a few others since then one way or another - and this made me a lot happier about shedding the physical ones that were now duplicated. Anyway, Calibre tells me almost 9000, but I know that I have not actually added everything to that collection.