Blond Tekikin wrote:

"The general approximation is 11-12 books per shelf foot."

My mother's old stunt was to have a double-depth bookshelves and shelve books behind books. (Since you're having them custom made.) Of course the books behind have to be things you don't want to access often & preferably are closely related to what is in front.
 

Yes, I’ve used that method. Part of it was to have room for all the books. But another was so as not to allow the shelves to metastasize all over the house, which happened anyway. This time, I’m fortunate to have a place away from everything else to be able to accommodate everything, without affecting the overall home appearance

Aja Jin wrote:

We have a small place (1500 sq ft) so we have shelves everywhere. To add some more, I had some (somewhat) inexpensive shelves custom made out of plywood, for paperbacks, to fit behind a door. It's deliberately simple. It's 55 inches wide, 85 high, 6 inches deep. We sanded and painted, and I love the way in turned out. It holds about 500 books.

Lovely! This would have been perfect, but we got the instructions wrong, and we upsized as we’re getting older. I think the instructions said “downsize” 😨