Generally for maths you can get two questions wrong and still have an 800 and for critical reading you can get about 4 or so wrong. It varies ever so slightly each year. You can find conversion tables in sparknotes and other websites, just search for it. For SAT IIs the table is more lenient, but then the content is also harder.
It's a lot harsher than UMS though. In one of my Further maths exams, i left out essentially a whole question (1 out of 8) and stil got 99% so i think it can be a bit silly. For SATs it's very easy to make mistakes because of the time constraints so its very difficult to get 800 even though you dont need full raw marks