Since I am writing a series, and planning on getting started on the next book right away, I found this blog post interesting: How Can You Write 4+ Books a Year and Maintain Quality Work?
The article isn't so much about how to maintain quality work, but how to set aside enough time to write a large number of books per year. While making the point that the same thing isn't going to work for every writer, the blogger gives some advice on getting into a writing routine (setting aside time to write so many words per day), putting book releases and other milestones on your calendar, writing about stuff that excites you (so that you're more likely to want to get it done), and hiring editors so that you can focus on the writing part.
I agree with most of the stuff in the post. Most obviously, if you're not excited enough to want to work on your novels every day, you should probably reconsider what you're writing — sounds like you're not cut out for it. And getting into a routine is great advice — something that I think NaNoWriMo and similar challenges really help with.
Hiring an editor is also necessary, I think, especially if you are planning on self-publishing — but I don't think I would do what this author does, and hire people to do all the editing for me. I think going through your own work and revising is necessary, especially since you might want to make huge changes to the story that an hired editor wouldn't or couldn't do. I plan to do several rounds of revision myself on each one of my novels, and only then will I have an editor come through to do a final check for problems.
And even then, I will probably read through it one more time myself.
Churning out multiple books a year is definitely possible, and it's also possible to maintain quality while doing so. But I do want to have a larger hand in revising and editing my own work, even if it means I finish fewer books per year.
What about you? Coming right out of NaNoWriMo, I imagine a good number of my readers are "wrimos," but if you do write throughout the year (and not just in November), how many books do you typically finish in one year? Or do you have a plan for how many you'd like to finish?