Here are some quick stats:
1) Population of Mexico per the 2017 census - 124 million
2) Percentage living below the national poverty line - 42%
That means, 52 million people live below the national poverty line (as defined by the Mexican government).
So for those who would craft immigration laws based solely upon morality or living a better life, our exposure is 52 million immigrants. I'd start there with Mr. O'Rourke and see what he would say. If he say's we would not allow that many over, then I'd begin counting down the numbers:
42 million
32 million
25 million
Tell me when to stop Mr. O'Rourke
20 million
15 million
Where is the line Mr. O'Rourke?
10 million
We need a number Mr. O'Rourke