Several of our technical titles have sold well over 20,000 copies but we've also had the rare title sell in the 2-3,000 copy range. That is definitely rare and only on very niche topics.
Writing a technical book is a lot of work on both ends, especially when a publisher provides real editorial and marketing support. Unfortunately, most no longer do, which is unfortunate for all involved and a good argument for considering self-publishing. After all, if publishers aren't offering added services, why not self-publish?
We read and edit every word before a book goes to copyedit. I've personally spent well over 300 hours editing individual titles. If you were to hire someone to do that work the minimum cost would be $15,000.
I'd love to see more people writing technical books -- good technical books -- for love of the subject and out of a desire to share knowledge. Sure, a good book will build your brand, but make it great, first.
The world always needs more good books. Our watchwords are "fewer, better books." Aim to be great.