"One in five?
More guessing?"
The number not apprehended per year can only be a guess, but so what? We know the number of illegals is growing each year, and the estimates we generally hear are some multiple of your cited 900,000, although I don't think I've heard anybody estimate 4.5-million. Maybe 2-3x?
So what?
The number is politicized by anyone because it's impossible to guess.
So let's say ten million...or maybe one billion, or even one gazillion!!!!