I would say 600 is about right. Actually, I expected a somewhat higher number - in the region of 1,000 or over.
After all, Sophia, Electress of Hanover (who lived at about the same time) left thousands of currently living direct descendants.
Admittedly, Louis' family was smaller and most of his children died childless. Only two of them had issue - Louis XVI of France (and all his children died without issue) and Charles X of France. Louis' descendants are through Charles Ferdinand, Duke of Berry, Charles X's younger son. Charles Ferdinand left a lot of descendants: for instance, Robert I, Duke of Parma (the son of Louise Marie Therese d'Artois, Charles Ferdinand's daughter) had 24 legitimate children, most of whom lived to adulthood and had issue (among them was Zita of Bourbon-Parma, wife of Emperor Charles of Austria).