There are many, many people out there who say they want to write a book. However, only a very small percentage of people ever actually write a book. An even smaller percentage will actually go ahead and polish and publish that book. And, you guessed it, an even smaller percentage of that percentage will go on to write multiple books to corner a niche and become truly successful.
This is actually kind of silly.
There are almost no barriers to entry these days. You could write a book today and publish it today, if you could write quickly and were familiar with the steps to publishing already. There are, in fact, authors who do keep up this kind of schedule, or very close to it.