I remembered answering this question way back, it turns out it wasn't a Top Ten,but a different meme.
1. Jane Austen
I own all six of Austen's novels, plus a copy of Sandition and Other Stories. I have three copies of Pride and Prejudice, three copies of Northanger Abbey, plus two each of Emma, Sense and Sensibility, Mansfield Park, and Persuasion. So that's...15 physical books.
2. C.S. Lewis
I own 11 of Lewis' books, including the Chronicles of Narnia and the Space Trilogy.
3. J.R.R. Tolkien
I own three copies of The Fellowship of the Ring, one of The Two Towers, two of The Return of the King, two of The Hobbit, two of The Silmarillion, and a book of short stories. So that's 11 physical books on my shelves. One LOTR set may technically belong to my dad.
4. L. M. Montgomery
I own 10 of Montgomery's books, including the entire Anne of Green Gables series.
5. J.K. Rowling
I own all 7 of the Harry Potter books.
6. Frank Herbert
I own all 6 of the original Dune books.
7. Madeleine L'Engle
Let's see, I own A Wrinkle in Time, Many Waters, Certain Women, Dragons in the Night, The Joys of Love, A Circle of Quiet-6 total. I've read many more of her books and hope to own them someday.
8. Ann Brashares
I have all 4 of the original Sisterhood of the Traveling Pants series, plus Sisterhood Everlasting and My Name Is Memory-6 books.
9. Louisa May Alcott
I have all 3 of the March family books, plus the two Rose books-5 in total.
There are other authors whose books I own two of, but I think those are all the bigger collections.