So, let's start at that very fine place to start, the beginning with "A" and the author you've read the most books from?
35 by Edgar Rice Burrows followed by a hefty amount of Stephen King and Tom Clancy.
35 by Stephen King, followed by 14 by Rick Riordan, 9 by Brandon Sanderson
35 by Lynn Kurland
I'm not even sure to count it but about 100 Nancy Drew books Carolyn Keane, followed by 33 L.M. Montgomery
About 63 books by Ann M. Martin
I'm definitely an Ann M. Martin too! I used to read a book day over the summers in Jr/Sr. High School
Until Jessica and Nancy told me their responses, I was overlooking the obvious choice in Ann M. Martin as well. I own over 150 of her Baby Sitter's Club books and well over 200 Sweet Valley High/University by the the various ghostwriters using the pseudonym Francine Pascal. If we're discounting pseudonyms, I've read 26 Michael Connelly novels.
But what about you? Comment below with the author, you've read the most books from and let's keep the conversation going!