I believe you have said** that $10K is common for an advance. Approximately how many books have to sell to earn out the advance?
It depends on what your royalty rate is. Royalty rates are negotiable so you'll need to know what your contract says before you can make these calculations. In other words, you won't know till you've got a deal in hand.
Let's use easy numbers to demonstrate.
You are offered an advance against royalties of $10,000.
You are offered royalties of 10% of jacket price for the first 10,000 books sold.
Assume your book will be published in hardcover at a book jacket price of $25.00
You will earn 10% of $25 for every book sold. That's $2.50
You'll need to sell 4000 books to earn out.
You won't be surprised to learn there are at least a dozen royalty rates in a standard publishing contract. Often there are many more.
One of the many things that have changed in publishing in the last twenty years are the proliferation of ways to sell books, each with its own formula and rate.
Your agent should be able to explain ALL royalty rates to you in ways you can understand. And s/he should be able to do the math for them too. If that's not the case, you need a new agent because auditing royalty statements is one of the biggest values an agent brings to the table.
**Well, I didn't say that but that doesn't matter for the question to be useful.