In a previous post, I described books as perpetual motion machines with respect to earnings, because once you publish them - especially ebooks - they have the potential to keep garnering sales forever. Furthermore, it's truly amazing how much (or rather, how little) your books have to accomplish in order to help you be successful financially.
Let's say, for instance, that you consider $50,000/year a comfortable income and would like to earn that from writing. That means you have to earn roughly $137/day from your books. Using KDP as the standard, if you sell your books for $2.99 you'll earn a 70% royalty (that's $2.093) from each sale. Thus, you need to sell 65.45 books per day to get to that $50K you want.
Now, if you only publish one book, 65 copies per day is probably a lot to ask of a single tome. Moreover, even if it does come out swinging and can hit that target initially, it may not be able to sustain that pace. However, if you publish two books, each only has to average 32.7 copies. Still a high hurdle, but a lot better than 65 for one novel.
Going a step further, imagine that you have 10 books published. At this juncture, your daily sales need only average a much more manageable 6.5 copies per book. And if you have 20 books out? The average number of sales you'll need per day for each book drops down to 3.27 - just a little over 3 books per day!
Selling books, however, is not a particularly easy chore, and I don't mean to make it sound as though it is. The entire process - from writing a novel to finally hitting the publish button - is an extremely challenging endeavor, and it only gets tougher as you try to find an audience for your work. In addition, publishing 10 or 20 books is likely to take a bit of time. It's not something you can do overnight.
Still, as the number of books in your backlog increases, the workload that each has to bear in terms of sales decreases. This, to me, is one of the essentials that indies must grasp: volume matters, and I think the odds favor someone who can get more of their product out into the marketplace.
Of course, I'm not saying that you should churn out trash just to ramp up the number of books you have for sale. Instead, my point was to show that you don't necessarily have to be at the top of the best seller list (or even on a best seller list) to earn a comfortable living from being an author. Once you get enough books out there, just averaging a couple of sales per day for each one will do wonders.
So, depending on what your goals are, if your books are selling something like 3 copies per day each, you may be well on your way to earning a good living from your writing. (Moreover, while it may not be easy, obtaining that level of sales is something that is certainly achievable.) You may never be a household name with that sales volume, but your own household is likely to be well taken care of - and you really can't ask for more than that.