I read an average of one book a week. That has been my pace for the last two years, which is far greater than it was in previous years. If I continue at this pace for the next forty years, that will come out to about 2,000 more books for the rest of my life. Assuming that I've already read 1,000 books to date (which is generous), that means I'll have read 3,000 books in my life. With almost 130 million books out there, that means there is roughly a 0.002% chance that I'll read any published book during my lifetime! If we confine the set of books to e-books, then we're talking about just over 1 million books. If all I read are e-books from now on, there is a 0.2% chance I'll read any given e-book (I'm excluding the 1,000 books I've already read, which are almost all printed books).
Of course, I'm not the only reader out there. Amazon hasn't revealed how many Kindles have been sold, but estimates range from 10 million to 20 million across all versions, including the Kindle Fire. Using the higher number and assuming that all Kindle owners read at the rate that I do, then I should expect 20 million * 0.2% = 40,000 copies of my book to be read by someone during the next forty years, an average of 1,000 per year. But that assumes every book is just as likely to be sold, which is definitely not the case. A Stephen King or James Patterson book is going to sell far more copies than a book written by an unknown author. If the top 100 books capture 50% of all book sales (I think it's actually more than 50%), then the rest of us are fighting for the other 50%. Therefore the odds of a reader like me buying my book is 0.1%, leading to 500 copies per year.
What I've left out of the many assumptions I've made is the growing number of e-readers sold, but I believe there will be a similar growth in e-books published, so those two cancel each other out. The other assumption is that the average e-book owner reads at the pace that I do. Based on the small sampling size of my personal friends, I'm almost certain that the average reader will read fewer than 50 books in a year. Even 12 books (just one a month) is a stretch for a majority of my friends.
The purpose of the exercise isn't to tell anyone how many books they should expect to sell. It was more of a mental exercise I went through to set my own expectations as an author. The point is, there are a LOT of books out there, and readers don't read that many books in their lifetime compared to the number of published books, so don't be surprised if you're not selling many copies.