A reader asked a while ago - Do you aim to have a certain # of books sold? How many books sold is considered a good selling book?
This largely depends on the publisher. A small press that doesn't pay advances, only pays royalities, and is print on demand might be thrilled if you sold 1500 novels. Other small presses might find 5,000 to be more satisfactory. Traditional houses in the CBA that pays advances and royalities and initiates a print run usually prefers sales to be more like 25,000 - 50,000 to be considered "good".
There are also the variables of whether or not this is a debut author (who obviously won't sell as many books as a well known, established author who has written 25 novels). Another variable to consider is the release date. Books that release in certain months are often less likely to sell as well as books released in the peak of the summer, etc. Holiday books will sell well in the month before and during their holiday (February for Valentine's Day, November for Thanksgiving, etc.) but three months later after the holiday has passed, sales are obviously going to drop. The editor will expect this.
Again, this is all guess-timating, mostly because this subject is taboo, as often times authors aren't even totally sure of their numbers - and they're usually even less willing to share them with others. Still, I think for the most part its safe to assume that selling 100,000+ of a novel is celebration worthy and guaranteed to turn heads. ;)
I wish I could give more specifics. Let's see - I know that with my Love Inspired novels, my third novel sold much better than the first and second ones, even though the second and third novels both sold in the top percent of books for that month. Yet because of the month's overall sales, the numbers were still vastly different. Does that make sense? Basically it means that February sold less books than April overall, so even though my books both did well in those two months, the numbers and $ were totally different. (I'm still waiting to get my royalty report to show me how sales did for my 4th novel - the Christmas novella compilation).
I hope this helps a little. Basically the answer is - no one knows ;) But once you are contracted and established in a publishing house, talk to your editor. Ask your agent the hard questions. Get as much info as you can so you know where you stand. It never hurts to ask! You will probably be respected for trying to learn as much as you can about your career.