I am wondering if top colleges (top 20) limit the number of admitted students by nationality?
ie. there is a maximum number of students from country x
It matters, and it definitely matters in financial aid awarding.
If you look at the numbers, the Ivies keep the overall percentage of international to about under 15% - so yes, nationality matters. I don’t believe there are strict limits on nationalities admitted, though obviously applicants from India and China, for example, face greater competition from their compatriots just because there is far more of them. Being from an under-represented part of the world helps, too: for example, sub Saharan Africa.
But make no mistake – these are still fairly extraordinary applicants with impressive credentials and accomplishments. Or children of presidents, kings and billionaires 
Yes. Harvard has a chart, you can see the number of undergrads (divide by 4 to see per year). All in all, there’s a certain number range for students from India, China, continental Europe, Africa.
To have an idea of the percentage admitted v. Number of applicants at these schools, take the RD percentage and divide in two, in three if you’re from an over represented country.