[piperclassique]

Well, more than 10,000, but probably less than 25,000.
Purchased about 800 to 1,000, and the rest are from various PD sources, mostly Gutenberg, but generous handfuls from Freereads and Manybooks, Internet Archive, U of Virginia, Baen free library, and various authors' sites. All the Baen CDs, and oddments from other free book sites.
Actually read, maybe 3,000 or so, including most of the bought ones. I expect to read a few more of the free ones, and will no doubt acquire and read more free and paid books in the future. Once I have them they stay at least in Calibre, except for the Gutenberg DVD, which I haven't transferred to the computer. I can usually download from the site more easily than finding the DVD and copying the file.
Errr. I have been reading ebooks since 1991, and tend to be a bit of a book hoarder. Our dtb library takes up the whole upper floor of the house, but at least half of those belong to my husband. Fortunately he now has a kindle as well.
Just looked in the Amazon Cloud. 1,239 titles since 2009. Mostly from Amazon, and about 75 % free books, already included in the above count. Quite modest, really.