Is there an average number of copies a picture book needs to sell before it's considered a success? How about a MG or YA novel? Or can a book's success be more accurately judged by whether it has earned back its advance? Or simply sold out its first printing? Please do not infer from this question that I've spent a lot of time lately staring at my royalties statements, as book sales mean not half as much to me as the smiles of my readers blah blah blah etc.You can think of a book's success as based on sales numbers. Certainly a book earning out its advance is something to be desired, but the advance and the print run are linked, and the print run is an idea of how many books the publisher hopes to sell in approximately a year. So the advance comes back to sales numbers, too.
Past that, though: No, there isn't an industry average for any type of book. Sales goals vary widely publisher-to-publisher and within publishers book-to-book. The thing to compare your sales to is the first print run.
sales are 1/2 or less of the first print run: This is a disappointment to your publisher. If the book was a small investment, the attitude in the office may be "ah, c'est la vie"; if the book was a very large investment, the attitude in the office may be "whose mistake was this, dammit, and whose neck is corporate going to wring?"
sales are around 3/4 of the first print run: Publisher response may range from "that's not so good" to "hey, that's not so bad".
sales are approximately the first print run: Publisher response ranges from "nice work" to "go us!".
sales are above the first print run: Publisher response ranges from "that's great" to "OMFG! Wearegeniuses!!".
sales bottom out: with the exception of a few very topical books, this is not expected and not appreciated.
sales dip, but are above 1/3 of the first year's sales: that's pretty normal.
sales are close to the same as the 1st year: awesome.
sales are above the 1st year sales: holy shit! quick, how did we do that? do it again!
book is still in print: congrats. have a bottle of champagne, because this is getting less common.
book is still in print: congrats! have a case of champagne, and invite all your friends over.
book is still in print: shh. stop celebrating, you'll just make the other authors bitter and envious.
(Also note that if your book's sales were not quite as high as your publisher hoped, but the book got some very positive review attention, that may still be chalked up as a "win".)
Let us remember, however, that one of your rights and privileges as someone not working in a publisher's padded cells is to distance yourself from the capricious mood swings, self-congratulation, and finger-pointing of the industry. Unless you fought your publisher through every step of the book-making process or in a fit of hubris took an advance that no book without an endorsement from God himself would ever earn back, then you can at most take a small fraction of the blame for a book's failure.
And unless the publisher is run by total jerkwads to whom panic and recrimination are as the air they breathe, your book's sales history will eventually be viewed with equanimity and perspective.
So as long as your books don't tank over and over again, things are probably just fine.