The debate between quantity and quality usually has to do with producing more books by sacrificing quality. Here's a different way to look at these two important Qs.
In the words of Ray Bradbury, "Quantity produces quality. If you only write a few things, you're doomed."
While I've only been publishing books since 2010, I've been writing since I was a teenager. The first full length book I wrote was absolutely terrible. It will never see the light of day. The short stories I attempted to write when I first started out have suffered a similar fate. I had notebooks and notebooks full of scribbled down story ideas that were only ever half developed. Yes, I started writing before the day when everyone had laptops and tablets. Everything I wrote in my teenage years died a slow death... except for one.
Escaping Fate was the first book I published, but it was actually written when I was sixteen. And then rewritten. And then rewritten again. It took me ten years from first, lousy draft to having something capable of entertaining a reader.
While I was rewriting Escaping Fate half a dozen times, I was working on other projects, improving my skills. One of those other projects was a mystery novel, which I seriously doubt will ever make it past my computer screen. Soft boiled mysteries are clearly not my genre. I don't know what possessed me to write a mystery novel. There were various other random things I wrote during that time. None of it amounted to anything, but it was great practice for me. Trying different lengths and genres helped me figure out what I really wanted to write... what I was actually good at writing.
Even once I started publishing books, nothing changed. I have 13 published books right now. Guess how many I've written over the past fifteen years. About double that. Some of them are still works in progress that need fleshing out or more researching or a total revamp. Others, completely fizzled out and will never be touched again.
Every book I've written, every book I've half-written, every book I've finished and then promptly trashed... they were all part of what Ray Bradbury is talking about. Quality and quantity isn't always about pumping out books by sacrificing quality. It's about writing good stuff and crappy stuff and mediocre stuff. It's about writing until you've honed your skills enough that you write more good stuff than lousy stuff. I don't think you ever get to the point where you only write good stuff, but that's okay, because it keeps you from getting lazy.