As a general rule, I believe in the old adage "less is more." You can do more and a better job when performing one task instead of spreading oneself too thin performing a variety of tasks. How many major corporations have lost their way by purchasing a wide range of companies and diversifying? After struggling to find their way in the touch and competitive world of business, these companies often sell off the divisions they purchased to diversify their business. In other words, they re-focused on their original business. Less became more.
In the world of self-publishing the opposite might be true, at least for the self-publishing author who is attempting to make a living in the publishing industry.
Why break with a principle that I believe to be true and not only practice, but encourage others to do so as well? For some simple reasons. I have no illusions that I can write a best selling blockbuster work of fiction that will hit the New York Times Bestseller's List and garner a movie deal in the process. But I do have considerable information to share about several non-fiction topics. None of these topics would appeal to a wide general audience.
If my goal is to make enough money to be able to quit "my other job" I am going to need to have multiple books selling one or two thousand copies a year. I am also going to have to keep researching and writing additional non-fiction books as the oldest titles on my self-published back list begin to decline in sales with the passage of time.
What is the perfect number of books to have in print as a self-publishing author? I do not know. I would imagine the given area of expertise and the amount of money a given author wants/needs to live for a fiscal year will be determining factor. If the author can write a book that will sell to a wide audience, it might be wise to focus on marketing that one single title and developing as many business opportunities as possible from that one book. For other authors it might take five or six books to generate the necessary income.
Regardless of the number of titles an author self-publishes, it takes lots of hard work and time.