The thing I want to key in on here is that word "prolific".
Ever notice that, in almost any other profession or endeavor you can name, being prolific—being efficient and productive, in other words—is a good thing … yet in Writing it is often looked down upon, even sneered at? After all, anyone who churns out an over abundance of words must be doing it hurriedly and sloppily, right? Gotta be a "hack", right? I mean, a true artist must suffer for his or her art, must endure sitting before a blank sheet of paper and "opening up one's veins" before ever hoping to produce anything truly meaningful. Isn't that so?
Loving what they do.
I'm hardly in the 300-plus books club. I likely wouldn't be if I lived another 65 years. But in the past few years I've gained a better understanding and appreciation for those who are able to reach such levels.
You see, from 1982, when I sold my first short story, until June of 2009, when I retired from my full-time job in the real world, I had written and sold six novels and twenty short stories. From mid-2009 until now, I have written/sold/had published an additional eleven novels and fourteen short stories; plus I have a twelfth novel completed and four more (two of them partially done) contracted/committed to have turned in by or before the third quarter. Obviously, no longer having that "other" obligation of a full-time job makes a difference; plus, some of the novels I've recently done—and some of the ones forthcoming—are more in the 40,000-word range rather the 60,000-plus ones I used to consistently do.
Nevertheless, by any measure, I have increased my output considerably. And you know what? I—along with a handful of first-readers whose opinions I solicit and trust—don't feel my quality has diminished at all; if anything it is better. I credit this to being more dedicated, more disciplined, and to working harder and steadier.
And the guys turning out 300-500 books in a career? I suspect they see themselves pretty much the same way and are fine with it.
Does a large quantity of titles automatically make a writer a hack? Not the way I see it. I could name several big-name, big-selling authors who do only one book every year or year-and-a-half that are automatic buys for the "beach book" crowd --- yet they can't write for beans. So, conversely, a modest output of titles certainly doesn't make the writer an author of any true significance.
The guys I'm talking about are merely writers who love what they do and work damn hard at it.
Nothing more. Certainly nothing less.