I read a little info blurb on Steven Erickson and it stated that he has sold 250.000 books from his Malazan Book of the Fallen series. That is only 25,000 per each book! He noted that he is surprised. that his book is mainstream fantasy. 250,000 books is mainstream? Even if you were to consider only the English speaking population, that sounds like s small number of book sold.
What is the idea number of books an author wants to sell to be considered successful?