I think when my son took it you could miss 5 or 6 and still get an 800.
I remember picking him up and asking how he did and he said "fine". Then I asked if he answered all of the questions and he said "of course not". I said "Wait, what?!? Why in the world would you not answer all of the questions?" He said "I answered enough to get an 800".
Sure enough, all of the questions he chose to answer were correct and he scored 800. A score of 800 was the 81st percentile for the January 2016 administration.
The Math I Subject test is much less forgiving. I believe he had to answer every question correctly to score an 800. A score of 800 was the 99th percentile for the June 2014 administration.
