Contrary to current wisdom, up to a point, the more people a country has the better. Free people (especially younger people) are resources. They produce more than they consume, and add vitality to an area. See for example Article about The Ultimate Resource
Attached is a projection of population by state, whereby each state with less than 1,000 people per square mile double their population density (number of people/land area), except for Alaska and Hawaii. They were left off because it is not clear how much of the land is habitable, and those with more than 1,000 people per square mile were reduced to that limit.
1,000 people per square mile was chosen because I have visited states like New Jersey and Rhode Island. They have more than 1,000 per square mile, and there seems to be lots of open area and green space. I also lived in Shanghai where there are about 9,700 people per square mile, and it wasn’t unbearable.
My analysis indicates that we easily could have 600 million people living in the United States. Doing so, our population density would be less than 170 people per square mile. Germany has about 600 people per square mile.
Opening our borders to all peaceful and honest people who wish to reside here is the humane way to go. Lets set an example for all the world to follow.
History has proven that all the warnings by earlier generations against Irish, Italian and eastern European immigrants were wrong. There is no reason to believe that the current concerns about South American immigrants will result in serious problems.