It’s easy to find out, if you have a Goodreads account. Well, at least it is if you’ve shelved all the books you’ve read and have entered a date read for each of the books.
On the my books page, near the bottom of the left column, is the link to your most read authors page. Clicking that link will give you a list of up to 100 authors. Ties are given the same number and no numbers are skipped. Click on the author’s name to go to the author’s profile page. Click on the number of books to see the books by that author you’ve shelved. It may display more books than the number reflects, because it shows all shelves and the number only counts the books with a date read.
Unfortunately it only counts the primary author, no co-authors or contributors are included. If you have lots of previously read books without a date read, you might want to at least guess at the year to provide more data.
It’s a fun statistic even though it’s not 100% accurate. You can also see this page for anyone who has a public profile. Which means they can see your list, too.