Okay, let me unpack this in a friendly way — the tricky part is that 'Mary Murphy' isn't a unique name in publishing, so there isn’t a single tidy number unless we narrow down which Mary Murphy you mean.
From my experience tracking down authors with common names, there are a few different possibilities: some Mary Murphys write picture books or middle-grade stories, others publish nonfiction or academic work, and a good number of Mary Murphys have contributed chapters to anthologies or co-authored books. If you count every single book, edition, translation, and contribution across all people named Mary Murphy, the total balloons quickly. If you mean one specific Mary Murphy (for example, the novelist or the children’s author), that person might have anywhere from a handful of titles to a couple dozen.
If you want a precise count, tell me one of her book titles, her middle initial, or the genre — I can walk you through looking her up on WorldCat, the Library of Congress catalog, Goodreads, or the publisher’s site and we’ll get an exact tally. Personally I like checking WorldCat first because it pulls in library holdings worldwide; after that I cross-check Amazon and Goodreads to catch self-published or small-press works. Give me a little more detail and I’ll help compile a clear list so you can start reading.