The easy answer is "enough to earn out your advance" if you're traditionally published, and maybe "enough to earn back your investment" if you're financing publication yourself through either hybrid or author-publishing.
The more complex answer, according to author and publisher Brooke Warner of She Writes Press, in this article Reframing Publishing Success in Publishers Weekly's BookLife, is an honest look at numbers.
While many authors state that selling 10,000 copies is their goal, Brooke cautions that “it’s an unrealistic benchmark for 95% of authors, and it’s especially unrealistic for debut authors.” She goes on to explain:
“In 2015, Lynn Neary reported a story on NPR called “When It Comes to Book Sales, What Counts as Success Might Surprise You” that noted that one of the books shortlisted for that year’s Man Booker Prize had sold fewer than 3,600 copies and another fewer than 3,000.”Her advice includes this gem:
“Debut authors would do well to think of their first books as an investment in themselves and their futures. It’s common book publishing wisdom that the needle doesn’t truly begin to move on book sales until authors publish their third book. As such, this industry requires patience, and selling 1,000 or 2,000 copies of a freshman effort is something worth celebrating.”And Brooke adds a reminder to:
“Celebrate the small victories, such as moments of connection with readers, a glowing review from a stranger, and the potential that these kinds of victories have to propel the next book.”It's an article well-worth reading.
Illustrate and Write On,\Lee