Huh?
I have a writing friend who writes romance and she tells me that Harlequin authors produce at least 3 books a year. I don't believe anyone can write 3+ books a year and not skimp on quality of writing.
Would a publisher expect a writer of YA Fantasy to produce more than 1 book a year?
I'm interested in writing both category romance and YA fantasy but can't churn out stories, so the answer to this will help me decide which type of book to try.
Three books per year is hardly superhuman writing. Nor is it "churning out" stories. Do you realize you just said that hundred of writers, many of them considered the best of all time, did not produce quality material? The number of classic writers who wrote faster than this, with a quill or fountain pen no less, is staggering.
Many of the best novels we have, some of them extremely long, were written in a month or less. Two of my favorite classic novels were written in fourteen days, and one was written in nine days.
Three romance length novels per year is not even very energetic writing, let along churning out stories. The average pro writer turns out about twice this much prose in a given year, even though it may be one or two really long books, rather than three or four or five shorter books.
I mean, really, get out the calculator. Writing three Harlequin length novels in a year means most writers will be working very little, and lazing around a lot. Three novels of this length per year really is a pretty lazy output.
There is no correlation between speed and quality. Some God-awful novels have been written very slowly, and some of the best novels out there were written very quickly indeed. Some writers can't produce quality material if they take ten years to write a novel, and others produce quality material every last time, even if they write a novel every other week, as some extremely good writers of the past actually did.
Never assume that others can't do something quickly and well just because you can't. All you have to do is look around a bit and you'll find hundreds of writers who do or did produce extremely high quality material at a very rapid pace. Much faster than three Harlequin length novels per year. And this almost certainly includes some of your favorite writers.
But as others have said, you're getting waaaaay ahead of yourself. First you have to write one novel that will sell, however long it takes, and most never get past this stage. If you do, then worry about how many books the publisher will want in a year.
The decision on what to write should not be based on how many novels a publisher will ask for, should you manage to write a first one that's publishable. It should be based or where your interest and talent lie, and on what you most love to read and write.