My shelves are a mess and I don't really have much of a system for the most part, but I reckon Iris Murdoch's going to win. Then again, having lots of Murdoch (or Lessing, I think) books is a bit like having a lot of Fall records, you can have about 50 of them and still only know a tiny amount of their output.
Going to try and do a count of a few of the big ones:
Simone de Beauvoir: 11 (4 volumes of her autobiography, 7 fiction, 0 philosophy)
Iris Murdoch: 9 that I could find
Doris Lessing: 7 (Doris Lessing I've managed to read all the way through: 1)
James Ellroy: 6
James Baldwin: 5
Joan Didion: 4 physical books, but one of those is a compilation of three books
Flannery O'Connor: 4 (plus an extra two by Flann O'Brien who isn't the same person but I keep reading their name as the same)
Shirley Jackson: 4 (plus a tiny little standalone copy of the Lottery)
Daphne du Maurier: 4
Jean-Paul Sartre: 4
Patricia Highsmith: 4
Margaret Atwood: 4
Mark Steel: 4
And then honourable mentions for Walter Benjamin, Victor Serge, F. Scott Fitzgerald, Olivia Laing, Hunter S Thompson, Jarrett Kobek, Virginia Woolf, Miranda July, and Kurt Vonnegut with a solid three entries each. I only own one physical Patrick Modiano book but that's another three-in-one compilation thing.