Where do I find the number of copies in a 1st printing of books by authors like Brandon Sanderson, Peter Brett, Brent Weeks, Patrick Rothfuss? Tried emailing them but did not get a response.
You're not alone in having a hard time finding those answers; the info just isn't easily accessible.
However, I have had occasional success finding the number of copies of a book by using Google search terms that include the author name and/or title name and the words "print run". In the publishing world, a "print run" refers to the total number of copies to be printed.
For example, I used the search terms "Brent Weeks" "Black Prism" "print run" in one Google search and found that the total number of copies printed of The Black Prism was 1500 books (straight from Brent). The trick is to include the words "print run" in the Google search. I was lucky in that found the answer on the first page of the search results just by reading the result summaries.
You also might try contacting the publisher for the information. Unfortunately, I haven't had any success with this method but hopefully you will. If you can get this info from them, you can be sure it's correct. You can find the publisher's name on the copyright page of a book.