My guess is the average technical book is probably in the hundreds of copies over a reasonable lifetime. Mine's sold 91 copies in 5 months. It's a book about how machine learning works. Unknown author, I have no social media and zero advertising budget. It's just Amazon organic listings. The economics are definitely not good but I didn't write it for money. I wrote it mainly for more existential reasons that I wanted to put my words out there and leave something behind in the world.