Trump wants 15,000 more agents on the southern border. The border is 2,000 miles. That means there would be 18 agents for ever mile of border or an agent for every 290 feet. This seems like a big overkill when you consider the difficulty in crossing large portions of the border and the technology we have available today to detect and track intruders.
Border patrol do not just patrol at the border. You'll find border patrol further in as well, sometimes, up to 70 to 100 miles away from the border. Maybe even further.
That's a lot of territory to cover.
I know when I worked out of corpus christi, our territory would take us to pharr Texas, pretty much to the border. There is a border patrol checkpoint about 60 miles north of the border, and you would see border patrol vehicles up to, and beyond falfurias, which is 73 miles away from the border.
There are a few of those border checkpoints on various highways, apart from the agents who patrol that whole area, and this is just in south Texas.