Last year it was the 10-year anniversary and as far as I know, it never sold out. The autodromo really has a lot of space. I don't remember the numbers too well, I think it had 40.000 people and 50k or 60k available tickets. Maybe somebody else remembers better.
Like anneirene said, you can buy the tickets online and then pick them up at a sales office. Just don't buy anything from non-official sites or people on the street. Should not be necessary to say that, then again I keep being surprised how people still fall for counterfeit tickets.