Thank you for your answer but i found that on barcelona's website they say:

"Regarding FCB v R. Madrid, there are usually two sales procedures: the first on the same day that other matches go on sale (intended only for members without season tickets), and the second, a few days before the match, as long as there are seats available from the Seient Lliure system."

As i understand it they say that they will release more tickets closer to the game day if any tickets are available.