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.
It's precisely because of how open the border is that we need so many agents.
Also, unless you're outright stupid , it isn't difficult to figure out. You have 3 shifts a day, plus everyone needs 2 days a week off, plus vacation time, sick time, personal days, whatever. Plus the fact that for every agent out actively "guarding the border" there are at least 2 others doing necessary office work, etc etc.
Let's use your numbers, 2,000 miles - 15000 agents and do some math.
First, let's simply divide by 3 (the number of shifts in a day0 and see that we only have 5,000 agents guarding 2,000 miles at any given time . Or 5 agents guarding 2 miles or 2.5 guards per 1 mile. Now consider that of those 2.5 guards per mile only 1 of them is actually out guarding anything, they other 1.5 is in the back doing paperwork and such.