I've read that only 2% of all books published today sell more than 5,000. On the other hand, I see a lot of authors, including self-published ones that claim that they make a decent amount of money from sales each month. I'd think that if you had a book published (either self or with a publishing house) and sold zero copies, you'd have to say that you were not a successful author. But how many copies do you think you'd need to feel satisfied? I've been wondering that myself - just how many before one can claim to be 'successful?' More than 50, I think, more than 200? and family and friends don't count - not that my family are at all interested in buying anything I wrote, though online friends have been very supportive. And still, a 5 star review from a stranger is an absolute thrill. No, I don't think I've anywhere near made it - not that it hasn't been fun.
Personally, I don't write for money, and I'm just as happy to see someone take a free copy of one of my ebooks off my website as pay for it, so my satisfaction comes from generating buzz and hearing other people 'like' my work (via reviews on amazon or goodreads or wherever). But I still feel very unsatisfied when I only get a few sales/downloads of my work.
Obviously the answer is going to be different for everyone. Stephen King would probably think a book that only sold 100k copies was a complete failure, but a self-published unknown author might be thrilled to sell 50 copies. How many copies of your book would you like to se sold?