The average book in the USA sells approximately 1000 copies over it's lifetime. This can range dramatically but if you can sell 500 copies of a book within a year then congratulations, you have beaten the average.
How well your book will do also dramatically depends on marketing presence, cover design, and your audience.
For exmaple, if you are writing a niche book on mushroom cultivation for urban homesteaders, you may have no problem selling to your audience because it is so specific. However, your audience is probably no more than a few thousand people total across the country, which does not give you very much room for growth.
Fortunately, here at -30- Press, we can help you monitize other services related to your book for additional income, especially if you are in a niche market.