There are around 250,000 books published by publishers in the US every year. The best number I could find on the internet suggested that there were 50,000 novels published every year. If 5% of these authors made a living, that would be 2500 self-supporting fiction writers.
However, other sources suggest that 70% of authors don't earn back their advance.
http://www.nytimes.com/2009/04/12/bo...w/Meyer-t.html This would mean that 30% of authors *would* earn back their advance. Average advances tend to be in the $30-40,000 range (with some obviously being much greater). This would mean that 15,000 authors would make at least that much money. (And the remainder would still get their advance).
So on this scale, 5% looks too low. However, it's not enough to get a, say, $50,000 advance; to make a living, you need to get a $50,000 advance *every year.* Or a much larger advance less frequently. So 5% may be an accurate percentage of those authors who can make enough from their writing to live off of it for a, say, 10 year period.