What is the real maximum score of subject math GRE scores currently? I know that every subject GRE test has the maximum score 990 and the maximum percentile 99. However, upon looking some data on the ETS website, I found that the math subject GRE score starts from 920. Also, I heard that somebody got score 930 and percentile 99. What is the truth? What is the maximum math subject GRE score?
I'm not a mathematician and not involved with this test; all I know is what I read on the internet. If anyone can confirm that they've seen someone get a perfect score in the last few years, that'd be great.

The maximum score is (almost certainly) 990. I assume you were looking at this, which shows that any scores 920 and above are in the 99th percentile; that is, only 1% of test takers score 920 or above. This does not (necessarily) imply that scores above 920 are impossible.

From the GRE website:


  Every Subject Test yields a total score on a 200 to 990 score scale, in 10-point increments. Note that each of the individual test scales occupies only a portion of the 200 to 990 score range.


The second sentence is a little vague, and in principle could imply that the top few scores are "not used." What we know for sure is that since 200-990 in 10-point increments is 80 possible scores, and there are only 66 questions, there will be at least 14 values that are not used. Since they said the scale is from 200 to 990, it would make little sense to "not use" the top few scores...but it's possible. 

One wrinkle: there is also this practice test in which a 66/66 only gets you a 910. But, I don't think we can necessarily extrapolate from that; it may just be that the practice test is easier than the current test.