As a data point, I've purchased (or borrowed from the library) 9 technical books this year. I've read bits and pieces of all of them, but I haven't read them cover-to-cover.
By then end of the year, I'll probably be up to 12-15 total, but not reading everything.
Edit: I probably read about 1/4 to 1/3 of each book (although this varies widely), so that puts me at ~4 full books.