A perfect score on the ACT is a composite score of 36. For 2017, there were 2,030,038 students that took the ACT. Out of those, 2,760 scored a perfect 36.
How rare was a score of 36? there are roughly 3,000 counties in the United States. That’s almost one student for every county.
As far as rarity, it’s much more rare to have a composite score of 1. Only one student was able to score a 1 in 2017. In fact, 3,161 people scored less than a 10. That makes scoring a 31 only slightly more rare than scoring less than a 10.